"I had a right to know...that I had a son, that I had a boy waiting for me, that my bloodline is sittin' in that truck!"
Mason was raised by his parents Jeff Koldyke and Violet Mason in Dallas, Texas. His father apparently left the family, and Violet took him to Las Vegas, New Mexico, where he spent his junior year of high school. His mother remarried to another man, who was a drunkard. Mason came to hate his current lifestyle in the town and began interacting with the wrong crowds. While in school, he became romantically involved with a girl named Tammy Mae Vasquez. However, their relationship was apparently strained due to his high-risk lifestyle and his sketchy friends. At some point during their relationship, he unknowingly impregnated her with his son, who she would name Cole. Mason then dropped out of school and ran away from home, later meeting Benjamin Wade and adopting his biker lifestyle. In 2009, the two began a crime spree together, starting with a robbery and mass murder at a burger shop that left four teenage employees dead. Then, they traveled to Texas, where they robbed an El Paso gas station and Wade killed the clerk.
However, they were arrested by police while trying to set fire to the station. Wade was given a lengthy sentence for his murder of the clerk, but Mason was sentenced to six years for his participation. They were both incarcerated at Texas State Prison, where they shared the same cell block as Lester Turner, who was imprisoned since 2004 for a rape and assault in Lubbock. Mason saved Turner from being shanked by another inmate, and they became close friends since then, with Mason apparently coming to neglect Wade in favor of Turner. They talked about opening their motorcycle shop in Mexico. Turner was eventually released in 2014 after completing his sentence, and one month prior to "Outlaw", Mason was also released. The two then opted to commit a crime spree in the same vein as the original followed by Mason and Wade before their 2009 arrest. All the while, Mason remained unaware that Tammy had given birth to their son, who she named Cole and left under the shared care of herself and her single mother Ruth.
Mason and Turner commit a robbery at Dee's Original Diner. As they steal the money, they take the three night-shift employees hostage and kill them. Afterwards, they both set the place on fire along with the employees. Mason and Turner then settle in a motel for the night. While Turner inhales drugs the next morning, Mason counts the stolen money. He then leaves the motel to see Tammy. Meeting up with her at her place, Tammy refuses to let Mason in and tells him to leave, but he refuses. Cole then comes in and addresses Tammy as his mother, causing Mason to realize that Cole is his son, and tries to confirm this by asking him how old he is. Tammy sends Cole away and tells Mason that she has a shotgun, threatening to use it if he doesn't leave, then forces him to leave. Mason returns to the motel and watches from the window as police cars pass by. When Turner returns, Mason asks him where he has been, and Turner reveals that he robbed a pharmacy, agitating Mason. Both of them later leave on their motorcycles.
Stopping by at a gas station, Mason enters the store to buy some food. However, the clerk recognizes Mason from the news and holds him at gunpoint with a shotgun. He tries to calm him down, but when the clerk takes out his phone to call the police, Turner steps in and shoots the clerk. Mason then proceeds to shoot out the security cameras while Turner robs the register. While scouting the store for more cameras, Mason spots a boy and his father and holds them at gunpoint. The father begs Mason not to shoot them, so he hesitates and lets them live, even stopping Turner from killing them himself. The two then make off with the money. However, the two eventually stop on the side of the road, and Turner asks him why he let the father and son live. Mason answers that he doesn't kill kids and that the boy needs his father. Turner promptly accuses Mason of becoming soft and deduces it is because of Tammy. Mason immediately decides that it is time for the both of them to split up for good. When Turner insults Tammy, Mason punches him and leaves him.
Mason returns to Tammy's house, but finds only Cole home, with Ruth babysitting. He takes Ruth hostage and begins playing with an oblivious Cole. Eventually, Tammy returns home and spots Mason. She tries to get Cole to come to her but Mason refuses to let him do so. He then continues to play with Cole, promising Tammy that he will make it up to her. After a while, Mason forces Tammy and Cole into Tammy's car and leaves Ruth behind. Driving into the nearby woods, Mason stops his car at a campground and steps out. Tammy approaches him, saying that she and Cole need to eat. However, a paranoid Mason tells her that they have to wait a while longer, then demands to know if Cole really is his son. Tammy confirms it, then realizes that Mason was behind the 2009 killings and the recent crime spree. Just then, Turner shows up, taunts Mason, and introduces himself to Tammy as Mason's partner. As Mason tells him to back off, the BAU and police show up and both sides engage in a gunfight. After Turner gets shot and killed, Mason sees Tammy and Cole being dragged away into a police car. Realizing he will never be a part of Cole's life, he continues firing and is shot several times by Morgan and Reid before being killed by Chief Raul Montoya.
During his robberies and killings, Mason always operated with a partner (initially Wade, but then switched to Turner following their releases). During the Burger Corral robbery, he beat, gagged, blindfolded, and restrained the night staff (all teenagers) right after closing hours (the males were tied up with their own belts and electrical wires, while the females were tied up with her own underwear), separated them by gender to establish dominance, raped the females, and killed them with single gunshot wounds to the head fired by a revolver. He then took their valuables and the money from the register and the safe, then set fire to the restaurant and the victims' bodies as a forensic countermeasure. He repeated this process during the Dee's Original Diner robbery, though this time, the victims (also teenagers except for José Rivera) weren't separated or blindfolded and were instead killed while facing each other, which was Turner's signature.
During Mason and Turner's robbery of the gas station, Turner shot the clerk while Mason shot out all of the security cameras as a forensic countermeasure, since they didn't have time to start a fire before the police arrived. During the campground shootout, he used both his revolver and a shotgun.
Mason is believed to be one of the unsubs responsible for the Burger Corral murders. A year after the murders, he was arrested along with Wade in Texas and while they were incarcerated, they met Turner. After Mason and Turner were released, their partnership was formed. It is believed that Mason returned to Las Vegas with Turner to emulate the original success he had with Wade. He is a methamphetamine user so it is likely the limbic systems in his brain have been damaged. This is disconcerting, because it can dramatically alter his behavior, give rise to a series of psychopathic tendencies, and eventually turn into an advanced psychosis. The dynamic between him and Turner is very rare, because it consists of two alpha males coexisting. They must have some kind of respect and/or fear that is keeping them together, but it is a tenuous bond. Eventually, a rift will grow between these two if it has not already.
Mason and Wade's 2009 robbery and mass murder at Burger Corral seems to have been inspired by a 1991 mass murder at a yogurt shop in Austin, Texas. In both cases, four teenagers were shot and killed inside a store that sold certain food, the girls were tied up with their own clothing and raped beforehand, and the store was set on fire afterwards as a forensic countermeasure. Multiple people were suspected in the 1991 case, with two suspects eventually being convicted and sentenced for the crime in 1999. However, the convictions were eventually overturned on the basis that their confessions were improperly used against each other. In 2009 (the same year the in-show Burger Corral killings took place), new DNA evidence found an unidentified man's DNA on at least one of the victims, which seemed to exonerate the two prime suspects, even though they knew details of the investigation that were never released to the public.