In The Benefactor, Mason, Violet and Garrett departed from the school bus and head toward school. When Garrett remarked that he didn't want to be home watching movies on yet another Friday night, Mason pointed out that they're freshmen and they just got off the school bus, which meant they won't be "hitting the clubs 'til 4AM." Moments after Violet insisted that they would be having a movie night at her house at 9PM before she and Garrett went inside, Liam arrived out front, looking pale and sweaty. Mason looked concerned and asked him why he wasn't on the bus with them, but Liam, who, unbeknownst to Mason, was in the middle of completing his transition from human to werewolf after being bitten by Scott McCall, lied and said that he ran to school. Mason didn't seem to buy Liam's story, since Liam lives three miles away from school, and he only became more concerned when Liam began to sway weakly on his feet. However, he didn't push Liam on the issue, and later had class with him.
After school, Liam had been tricked by Kira Yukimura into coming to a "party" at Lydia Martin's family's lake house, not knowing that, in reality, the McCall Pack intended to give him a supernatural intervention before that night's full moon. Mason texted Liam while he and Kira were on their way to the lake house to ask him where he was, and Liam, not yet realizing that it wasn't actually a party he was about to attend, invited Mason to the lake house as well. Mason, in turn, invited Garrett and Violet, which ultimately led to at least fifty freshman showing up at the lake house for a party that wasn't actually planned to occur. Just as the moon hit its peak, Scott and Kira took Liam into the boat house while Stiles and Malia went to the basement, leaving Lydia alone to host the impromptu party.
Mason, Garrett, and Violet led the large crowd to the door, which Lydia answered. When Mason asked if they were in the right place for the party, Lydia replied, "Absolutely" and ushered them in. Mason immediately began looking around for Liam, even going upstairs despite Lydia's orders for everyone to stay downstairs. Lydia followed him into the upstairs study and instructed him to go back to the party, but Mason insisted he was just trying to find Liam. Lydia, getting overwhelmed and frustrated with having to cover for her supernatural friends, retorted that "missing freshmen" were low on her priority list, which led Mason to point out that she clearly cared enough to know he was a freshman. Rolling her eyes, Lydia lied and said she might have seen him downstairs while she picked up two abandoned wine glasses, but as she tried to usher Mason out of the room, she accidentally spilled the wine on the pristine white carpet of the study.
Lydia began to cry as she frantically tried to mop up the wine with her sweater, and Mason looked extremely guilty as he apologized for what happened. When he asked her if the carpet was valuable, Lydia confessed that the problem was that nothing in the lake house was valuable, and that she needed to keep it perfect because she and her mother were selling it and needed to get every penny they could out of it. Mason, feeling even worse now, comforted Lydia, assuring her that he would go get club soda and salt to get the stain out, and that it would all be okay.
In I.E.D., he and Liam were in the weight room working out, with Liam using the bench press and Mason doing bicep curls with hand weights. While they exercised, Mason mentioned that Garrett had borrowed his hoodie and never returned it before stating that he went to the new housing development on Spaulding where Garrett he lived. When Liam didn't understand Mason's point, Mason revealed that the housing development was "still in development," and that unless Garrett was living in a backhoe, he had lied about where he lived.
Mason then used this story to segue into his concerns about Liam, bringing up the fact that his best friend was doing uncharacteristically strange things like running three miles to school and disappearing at parties. It was then that Mason realized that Liam had nervously added nearly 400lbs to the bench press' barbell and wondered aloud if Liam was on steroids before asking him if he really intended to lift it. Liam, alarmed by the fact that Mason was getting suspicious about his new supernatural traits, immediately stopped what he was doing, and was slightly relieved when Mason mistakenly attributed Liam's strange behavior to the upcoming lacrosse game. Liam explained that he wasn't worried about it, since it was only a scrimmage, but when Mason reminded him that they were playing Liam's old school, Devenford Prep, Liam became anxious.
After school, Liam and Mason approached the Devenford Prep bus as they pulled up to Beacon Hills High School. When Liam's former rival Brett Talbot and his friends departed the bus, Mason became nervous as the Devenford players came off the bus and immediately began taunting Liam for his behavior toward their coach, which is what got him kicked out of school. Mason became even more concerned when he saw Scott and Stiles show up and usher Liam into the locker room before things could get violent. At the scrimmage, Mason was on the sidelines with Liam just before the match started, and when Brett began changing into his uniform on the other side of the field, Mason couldn't help but check him out. When Liam realized what he was doing, he called Mason out on finding Brett attractive before pointing out that Brett wanted to destroy him. Mason attempted to deny his attraction to Brett before eventually retorting, "I think you could definitely take him... And then give him to me."
Afterward, Mason joined Violet in the bleachers and watched the game with her, helping her hold up signs for Liam and Garrett. He was horrified when he saw a very bad tackle between Brett Talbot, Garrett, and Liam, the former of whom was, unbeknownst to Mason, cut with a wolfsbane-laced blade hidden inside Garrett's lacrosse stick.
In Orphaned, Mason and Liam went on a run through the Beacon Hills Preserve behind the high school, where both boys were clearly rattled by the events of the previous night, albeit for different reasons. Mason nervously rambled to Liam about the fact that it had been revealed that Violet and Garrett were actually professional assassins who were only befriending them as part of their cover. Liam, who was concerned about the Deadpool in general but couldn't tell Mason this, remained quiet and focused on the run, and when Mason demanded to know how he wasn't more freaked out about this revelation about their so-called friends, Liam insisted that he was worried about a lot of things before taking off on a dead sprint. Thanks to Liam's new super-speed due to his werewolf transformation, Mason couldn't keep up with him, and though Mason shouts at him to wait for him, Liam kept going.
Before Mason was able to catch up with Liam, Liam was hit by Garrett's SUV and stabbed with his wolfsbane-laced blade before he was captured. Mason stopped right around where Liam had been nabbed and looked around, but couldn't find him anywhere, not noticing the small puddle of blood near his feet. After realizing that Liam was gone, Mason returned to the high school and sought out Scott McCall to see if he had seen Liam, since he had noticed the two had struck up a friendship. When Scott revealed that he hadn't seen Liam either, the two went to talk to Coach Finstock, who was in the middle of chugging a bottle of cold medicine and who informed them that Liam had skipped his class. When Coach suggested that perhaps Liam was sick as well, Mason told Scott that he hadn't looked sick on their run. Just then, the late bell rang, which led Scott and Mason to agree to let the other know if either of them ended up finding Liam before they went their separate ways.
In Perishable, Mason caught up to a visibly-panicked Liam (who had been experiencing post-traumatic flashbacks to his battle with a Berserker several days earlier) at school. Mason asked Liam if he planned to go to the bonfire that night, and when Liam admitted that he was probably going to skip it, Mason reminded him that he couldn't skip because attendance was mandatory for members of the lacrosse team. Mason continued to encourage Liam to go to the bonfire by outlining his plan for them; namely, he wanted to help Liam find a "nice girl he can embarrass himself in front of" and then find Mason a hot lacrosse players, since, in Mason's words, "Statistically speaking, someone on your team has gotta be on my team, right?" Liam, on the verge of another flashback-induced panic attack, quickly agreed to go to the bonfire with Mason before he left for class.
That evening, Mason and Liam sat together at an outdoor picnic table during the bonfire, where Mason watched Liam fill up his soda bottle with a bottle of vodka hidden in a brown paper bag. Though he's somewhat concerned by how heavily Liam seemed to be drinking, Mason insisted that he wasn't going to warn Liam to slow down; on the contrary, Mason insisted that he wanted him to get "blackout, head in the toilet" drunk so that maybe then, Liam would be more inclined to tell him the truth what has been going on with him lately instead of continuing to lie. When Liam started to clam up, Mason insisted that he wasn't asking simply because he wanted to know, but because he wanted to help, and though Liam seemed to appreciate Mason's concern, he assured him he was fine and left to get more alcohol.
Later on, Mason became even more worried when Liam began acting much more drunkenly than he should have been based on how much alcohol he had drank, and he eventually met up with Scott and Malia, the latter of whom was experiencing the same symptoms despite only having a few sips of vodka from a flask. When Scott asked Mason how much Liam had to drink, Mason insisted that he didn't drink enough to be barely-conscious and stumbling around like he was, and Scott became very concerned about what was happening. However, when he started to tell Mason that they all needed to go, Scott began slurring his words, leading Mason to ask how much alcohol Scott himself had drank. When Scott assured him that he hadn't had a sip (which was true, since, as a werewolf, Scott can't get drunk anyway), they both became even more worried about what was going on. After a moment, Scott realized that somehow, the subsonic frequencies in the music the DJ was playing was affecting him, Liam, and Malia due to their werecreature-enhanced hearing. Unable to tell this to Mason, Scott simply told him that he needed to turn off the music and ordered Mason to not let Liam and Malia out of his sight.
Unfortunately, shortly after Scott left to head to the DJ booth to shut off the music, the DJ turned up the volume, sending Scott to his knees and weakening him enough to allow the security guards to capture him. Afterward, the other guards went after Liam and Malia, and when Mason insisted that they were his friends and was taking care of them, the guard claimed that Liam and Malia were overly intoxicated and needed to leave. Mason offered to go with them, but the security guard, having ulterior motives, shoved Mason onto the ground before dragging Liam and Malia into the high school where the others had brought Scott. Remembering Scott's words about the music affecting the three, Mason walked over to the DJ booth and found the cord that hooked up to the speakers before yanking it out of the plug. Though Mason was unaware of just what he had done, he had inadvertently saved the lives of Scott, Liam, and Malia, as the security guards had doused them in gasoline and was about to set them on fire when the music was cut off, allowing Scott to steal the lighter away from the guard in time for Braeden and Derek Hale to take out the guards.
In A Promise to the Dead, Mason was playing games in Liam' bedroom, where Liam seemed to be dominating him. Mason wondered aloud how Liam became so good so quickly, and asked him if it was because he had been practicing or because he had somehow developed superhuman reflexes. Liam nervously insisted that he had just been practicing, not wanting Mason to find out that he is a werewolf, and Mason seemed to accept this answer before standing to his feet and preparing to leave, citing the fact that they have a history test the next day. Liam, who was still having flashbacks and panic attacks to his battle with the Berserker, pleaded with Mason to stay for one more game, but Mason reminded him that he had already been talked into "one more game" four times and that he really needed to study. Not wanting to be left alone, Liam instead suggested that they could study together there and that Mason could stay the night, which led Mason to sense that something was wrong with Liam. However, when Mason asked Liam once again if he was okay, Liam began to clam up and insisted that he was fine before reluctantly stating that he would see him the next day. Mason seemed to not believe Liam's insistence that he was fine, but he ultimately left to go home anyway.
The next day, Mason and Liam went to the weight room as usual, where Liam, who had been up all night having flashbacks about the Berserker, seemed tired and irritable. When Mason noticed just how much weight that Liam was putting on the bench-press bar, he offered to spot Liam, but Liam somewhat rudely declined and instead asked Mason, who was only holding eight-pound handbells in each hand, if he needed a spot, leading Mason to retort, "This is my warm-up, dick." When Liam prepared to start bench-pressing, Mason walked around the bench and prepared to spot Liam anyway, claiming he didn't want to see Liam kill himself with too much weight. Now very annoyed, Liam did two reps in front of Mason before snidely asking him if it looked like he needed help. Realizing that something was clearly up with Liam, Mason backed off and went across the room to work out by himself, leaving Liam alone on the bench-press. Though his first few reps went as planned, he eventually had a panic attack caused by flashbacks of the Berserker battle, which temporarily blocked his super-strength and prevented him from lifting the weight from his chest.
Mason didn't notice Liam's struggle until Scott, who had sensed something was wrong with Liam through his werewolf senses, rushed into the weight room and pulled the bar off of his chest, allowing Liam to sit up and pant for breath. Mason watched in horror at the fact that Liam had nearly hurt himself badly, and was confused when he overheard Scott whisper, "If you don't want to be with us, that's okay. But don't push your friends away, too." That evening, Mason showed up in Liam's room with a stack of video games and stated that he knew something was going on with Liam, and that he's well aware that he doesn't want to talk about it. He went on to say that Liam was his best friend, and that since the last friend ended up being a professional killer, he didn't have many option for new ones. Mason then added that he planned to wait until Liam was ready to tell him what was going on, and that until then, there had to be a video game in that stack in which Mason could "kick Liam's ass." Liam laughed and chose a game he hadn't yet played before the two began to play.
The next day, Mason attended the lacrosse game with Sydney, once again holding up signs cheering on Liam, who was clearly struggling with his panic attacks and the upcoming full moon, though he was eventually helped by Brett Talbot.
In Smoke and Mirrors, Mason was at a study group at the school when Liam (who was about to leave for Mexico to rescue Scott and Kira from Kate Argent) called Mason to ask him to look for Lydia Martin, who was also at the high school and who wasn't replying to their phone calls. Mason, still on the phone with Liam, saw that Lydia's car was still in the parking lot of the school and agreed to call him back as soon as he found her.
When Mason went back inside the school, he looked around several of the empty classrooms before finding Lydia's phone and the jacket Lydia had retrieved from Kira's jacket laying on the staircase. He muttered to himself that this must be why Lydia wasn't answering Liam and the others' calls and texts before picking up the phone to find a blurry photo of a Berserker had been set as the lock screen as a warning/clue from Lydia. Before Mason could wonder what was going on, the Berserker appeared and backhanded Mason so hard that he was knocked unconscious before he was dragged downstairs to the basement where Lydia was also being held hostage.
Mason awoke in the basement with a bleeding wound on his temple and a bad headache to find Lydia kneeling next to him to check on him. Mason was horrified to find that the Berserker had taken his phone, but Lydia stated that this was the point, since it clearly didn't want them to call for help. Mason, who still knew nothing about the supernatural, began describing the creature as "a man with a bear skull over its face," but when he demanded to know what it was, Lydia simply replied that it wasn't human. Lydia, who knew that the pack was on their way to Mexico to save Scott and Kira, wanted to make a break for it and asked Mason to check and see if the Berserker was still guarding the hall, which Mason confirmed that it was.
When Lydia, desperate to save her friends, found a container of baseball bats and decided to go after the Berserker, Mason asked her if she was crazy, leading Lydia to explain that her friends were in danger, as was Mason's best friend Liam. This seemed to shock Mason into action, and before Lydia could rush after the Berserker, Mason had grabbed a baseball bat of his own and lunged for the Berserker with a battle cry. Mason only got in several hits before the Berserker threw him backward, hitting the wall at the other end of the hall, and Lydia was only able to hit it several times as well before she was thrown backward and landed in a heap next to Mason. When the Berserker stormed toward them, Mason brought up the fact that Lydia had said that it didn't want to hurt them, it just wanted to keep them from leaving, Lydia shrieked that it seemed to have changed its mind.
Fortunately for them, Sheriff Stilinski arrived just then and called out, "Excuse me!" causing the Berserker to briefly forget about Lydia and Mason as he turned toward the Sheriff. Sheriff Stilinski then tossed The Mute's unused claymore mine at the Berserker, who caught it and looked at it just as Sheriff dove into the adjacent hall, waiting until Lydia and Mason quickly took cover behind a bookshelf before pushing the button on the remote detonator. Once the dust settled, Mason, Lydia, and the Sheriff climbed out of their hiding spots and gathered in the hall, where they found that the Berserker had been turned into a pile of dust with a bear skull on top. Lydia revealed that she believed this was all some kind of distraction in order to prevent her from catching up with the others, and Mason looked alarmed and confused when Sheriff seemed to know exactly what she meant. When Sheriff guessed that it was because Lydia knew someone was going to die, Lydia nodded grimly in confirmation. Mason, even more alarmed now, asked her who was going to die, and Lydia was near-tears when she replied, "Scott."