Born and raised in Alaska, the geographical proximity of his home state to Communist Russia brought the threat of the Cold War that much closer to his doorstep than most Americans. This was a key contributor to his fervent anti-Communist ideals and eventually led to his enlistment in the USMC. He spent his youth hunting elk and grizzlies in the Alaskan wilderness with his father (a WW2 veteran and recipient of the Purple Heart for wounds received during action at the Makin Atoll); as such, he is adept in cold weather environments and is an excellent sniper (at the age of 20 he became the youngest Wimbledon Cup winner in the history of the marksmanship competition). He was recruited to SAD/SOG at the age of 25 after an exemplary term of service with the Marines. Having served with distinction since joining the Agency, Mason has a long standing friendship with many of his fellow Agency colleagues and shows a particular kinship with operative Frank Woods. Always itching for action, Mason's tendency towards impulsiveness is the one trait that may not have been completely "trained out of him." However, his close friend Woods is well aware of this, and knows exactly when and how to rein him in.

Prior to joining the CIA, Mason served as a U.S. Marine in Afghanistan. On November 29, 1957, Mason is pinned down in a village by Afghan soldiers during routine patrol, with seemingly no escape. Here, he encounters Soviet defector Yuri Raslov, who assists him in escaping the village in a stolen truck, and arrive in an allied camp. Shortly after these events, Mason is recruited into the CIA.

In 1961, Mason is a member of a CIA task force known as Operation 40. Mason and fellow Operation 40 membersJoseph Bowman and Frank Woods infiltrate Cuba prior to the Bay of Pigs invasion to carry out Operation Zapata, the assassination of Fidel Castro. Using the distraction caused by the Bay of Pigs invasion, Mason, Bowman, and Woods raid a compound where Castro is reportedly living. Mason and Woods encounter Castro with a mistress in his bedroom; Mason shoots and kills Castro. During their escape, Cuban military forces rapidly turn the tables on the U.S.-supported Cuban rebels and swarm the airfield where Op 40's escape plane is located. While Mason, Bowman, and Woods make it to the plane, Mason bails out just before takeoff to clear the runway of Cuban military vehicles. Mason is subdued and captured by Cuban troops. Regaining consciousness, Mason watches as Fidel Castro, alive and well (the "Castro" Mason shot was a body double ), turns him over to Soviet General Nikita Dragovich as a "gift".

Mason is transported to a Soviet labor camp at Vorkuta, Russia. There, Dragovich had Friedrich Steiner brainwash Mason; Mason is implanted with the knowledge to interpret the outgoing broadcasts from Dragovich's numbers station, and is also programmed to become a Soviet sleeper agent with the primary purpose of assassinating U.S. President John F. Kennedy. At the same time, however, Viktor Reznov, a former hero of the Red Army who was betrayed by Dragovich and his underling Colonel Lev Kravchenko, also brainwashes Mason, programming him to kill Steiner, Kravchenko, and Dragovich out of revenge. Sometime after his brainwashing, Mason and Reznov plot their escape from Vorkuta. On October 6, 1963, the breakout succeeds and Mason escapes back to the U.S.; Mason remains unaware he was ever brainwashed.

Upon his return to the U.S., Mason is made a member of the CIA's Studies and Observation Group, which also includes his former teammates Woods and Bowman. On November 10, 1963, Mason is escorted by his new handler Jason Hudson and U.S. Secretary of Defense Robert McNamara to the Pentagon, where he is given direct orders from President Kennedy to begin Operation Flashpoint: Mason and his team must locate and kill Dragovich and sabotage the Soviet space program. Throughout the meeting, Mason recalls experiencing everything as though it was a dream; when speaking with Kennedy, Mason hallucinates taking a pistol and pointing it at Kennedy.

On November 17, 1963, Mason and the S.O.G. infiltrate a Soviet Cosmodrome in Baikonur, Kazakhstan per Operation Flashpoint. The Soviets are already aware of this attack the capture of Grigori Weaver, a CIA double agent who was compromised by Kravchenko. Mason and his team are able to rescue Weaver and fight their way to the control bunker for the Soyuz 2 rocket, but are unable to abort the launch. Mason uses a Valkyrie Rocket to destroy Soyuz 2 in mid-air, then the S.O.G. begins a sweep of the facility to eliminate the surviving members of the Ascension group, a group of former Nazi scientists who had been collaborating with the Soviets, and to hunt down Dragovich. They corner Dragovich in his limousine and destroy it, but rather than verify Dragovich's death, Mason decides to move on after being told by Woods that the explosion wouldn't have left anything recognizable. The S.O.G. assumes Dragovich is dead, but this is later proven to be far from the truth.

Almost five years later, in 1968, Mason's SOG unit was sent to Vietnam for the purpose of searching for covert Russian operations in Vietnam and Laos. After landing at Khe Sanh to meet up with Woods and the rest of his old unit, the hill of Khe Sanh comes under attack by NVA forces. Mason, Woods, and Special Agent Hudson repel the beginning of the attack before the SOG leaves for its mission.

In Hue City, they attempt pick up a Soviet defector with vital information of Soviet involvement in the war. Inside a safe room, Mason believes he is reunited with Reznov, "the defector", although "Reznov" is a figment of his imagination and the real one died after Mason got on the train from Vorkuta. After collecting the intel, the team leaves Hue.

While heading north into Communist territory, Mason's helicopter is shot down but his life is saved by fake Reznov. Mason and the rest of the team silently enter a village where they sabotage Vietcong equipment. After, Mason and "Reznov" clear out a rat tunnel and retrieve a Soviet operations book from Kravchenko's office. The tunnel begins to collapse and Mason barely escapes with his life.

The intel leads them to a Soviet cargo plane downed in Laos which was believed to be carrying some sort of biological weapon. Mason remembers stories Reznov had told him about a horrible nerve gas called Nova 6, originally developed by Nazi scientist Friedrich Steiner, but commandeered by General Dragovich and reproduced by Daniel Clarke. After combat up the river to the crash site, SOG is set upon by both VC and Spetsnaz before making their way into the plane. There, they find the Soviets have already retrieved the cargo and were waiting for them to arrive in the valley below. After an explosive fight, the plane wreckage tumbles down a cliff and the team is taken prisoner by the Spetsnaz.

At a VC compound, Mason, Woods, and Bowman are forced to play Russian Roulette. Bowman defiantly insults his captors, prompting their Russian contact to bludgeon him to death with a pipe. Woods plays and thankfully chooses an empty chamber, although he whispers to Mason that the next one is loaded. Mason shoots the VC leader in the throat and takes his pistol, fighting out of the compound and killing the Russian. Woods and Mason commandeer a Hind and fly it to Kravchenko's compound, where they battle to his office along with "Reznov" and attack him. Mason is overpowered and Kravchenko begins to repeatedly kick him in the face, but he is stabbed by Woods. In his last breath, Kravchenko pulls the pins on his grenades, but Woods throws Kravchenko and himself out of the office window, saving Mason.

A letter in Kravchenko's office shows Steiner is overseeing Nova 6 operations on Rebirth Island in the Aral Sea. Mason infiltrates the labs at exactly the same time the CIA is battling through the island village. Special Agent Hudson finds the office of Steiner seconds too late, as Mason has already reached Steiner and is getting ready to execute him, as he thinks he is Reznov. The CIA captures Mason after he shoots Steiner in the head and brings him back to the United States.

Following his apprehension by Hudson and Weaver at Rebirth Island, Mason is interrogated ruthlessly. With Steiner's death, Mason is their only link to the numbers station and the impending Nova 6 attack by sleeper agents in the capital cities of every state in the U.S. In addition, the President was prepared to launch a full nuclear strike against the Soviet Union in retaliation. If the numbers station could not be located and neutralized before Dragovich sent the signal to attack, World War III would break out.

After forcing Mason to relive his experiences leading up to the present date, Hudson had yet to learn anything from Mason. With DEFCON 2 declared, Hudson reveals himself to Mason out of desperation and removes his restraints. Mason knocks Hudson out and makes his way through the interrogation facility, remembering his brainwashing at Vorkuta, realizing that all of his encounters with Reznov after Vorkuta were imagined, as Reznov died during the breakout, and finally understanding the meaning of the numbers broadcasts. After being subdued by Hudson, Mason explains that the numbers station is on the Soviet cargo ship Rusalka, which Mason had originally seen during his first meeting with Dragovich in 1961.

Mason, Hudson, and Weaver raid the Rusalka, located in waters southwest of Cuba. In the cargo ship's hold, they discover an antenna with a tether leading into the depths of the ocean. While a U.S. Navy fleet is en route to destroy the Rusalka, Mason insists on entering the submerged numbers station to kill Dragovich once and for all. After fighting through the numbers station as it comes under fire, Mason finds the console controlling the numbers broadcasts and attempts to deactivate it, but is knocked away by Dragovich. Hudson comes to Mason's aid, distracting Dragovich and allowing Mason to pull him down. Mason strangles Dragovich, then escapes the imploding numbers station with Hudson. With the station destroyed and Dragovich dead, the crisis is averted.