|Plot details follow, read at your own risk.|
- "You know what I'm capable of and yet you walk in here, like lambs dressed for the slaughter."
- ―Garrett Mason
Mason worked as an agency temporary bartender for various bars throughout Los Angeles. However, his work as a bartender was a cover for his career as the "Werewolf" serial killer, more infamously known as the "Black Dahlia Killer". Mason was the one who killed Elizabeth Short, the infamous and unsolved "Black Dahlia" murder of 1947. As Short's murderer, he also presumably had a medical background. Six months later, Mason resumed his grisly string of murders during LAPD Detectives Cole Phelps' and Rusty Galloway's time in the Homicide Department.
While working at the bars, Mason observed and targeted several women, all of whom shared traits such as emotional problems, difficult relationships and/or marriages, and varying degrees of alcohol abuse. The women would usually get drunk and talk to Mason about their problems, giving him information that he would later use to frame different people for his crimes. Mason preyed on and brutally murdered Celine Henry, Deidre Moller, Antonia Maldonado, Theresa Taraldsen, and Evelyn Summers. He also framed Alonzo Mendez, Eli Rooney, Clem Feeney, Stuart Ackerman, and Grosvenor McCaffrey. Mason beat and strangled his victims to death. He also left several of them stripped naked with cryptic messages written on their bodies, either in lipstick or scratched on, and finally stole pieces of jewelry (such as Elizabeth Short's social security card, Theresa Taraldsen's shoe, Evelyn Summers' ring, Celine Henry's ring, Deidre Moller's watch and Antonia Maldonado's medallion) from the victims as trophies for each murder. Alonzo Mendez was framed for Celine Henry's murder, Eli Rooney was framed for Deidre Moller's murder, Clem Feeney was framed for Antonia Maldonado's murder, Stuart Ackerman was framed for Theresa Taraldsen's murder, and Grosvenor McCaffrey was framed for Eveylen Summer's murder.
Mason intentionally varied his methods with each victim, but always left clues to link all the murders together. Mason's intent behind this was to test if anyone could connect the murders to one another and to him. After committing each murder, Mason took evidence from the crime scenes, such as the murder weapon and personal effects of the victim, and was able to frame other people who were closely linked to the victim, as it was generally assumed that his murders were being committed by Black Dahlia copycats.
Events of L.A. Noire
- "The newspapers will crucify the guy. They won't have time to wonder about our mistakes."
- ―Cole Phelps
- "The LAPD, you say? Is it really possible that you could have found me after all this time? How interesting."
- ―Garrett Mason, after he was confronted by Phelps at the Christ Crown of Thorns church
The final clue led Phelps to Mason's hideout at the Christ Crown of Thorns Church, where Mason awaited Phelps' arrival, armed with a shotgun. Phelps recognized Mason as the temporary bartender at The Bamba Club during his work on the Henry case. Mason in turn applauded Phelps for having the tenacity to find him. Mason then fled into the church catacombs with Phelps in pursuit. During the chase, Mason taunted Phelps by asking why he joined the LAPD, while alluding to Phelps' seeking for personal redemption. The chase concluded in the church's graveyard where Phelps shot and killed Mason.
- "You got no one. Mason was a ghost."
- ―Captain James Donnelly
Despite being the Black Dahlia killer and even with compelling proof of his murders, Captain James Donnelly revealed to Phelps and Rusty that Mason was actually the half-brother of a very powerful and undisclosed politician, hence Mason's name and acts were to be kept away from public knowledge and out of official records. The suspects that were previously arrested were quietly released and the truth was quietly buried to all but a handful of men.
During the Arson DLC Case Nicholson Electroplating, Herschel Biggs asked Cole who the real Black Dahlia killer was, but Cole refused to answer the question, saying he made a promise to the department. In addition, it is possible to hear passers-by comment on a possible 'coverup' over the Black Dahlia killings, suggesting that rumors about Mason may have been leaked.
- "Not sure. Medium height, medium build... dark hair, I think. Sorry. He just had one of those forgettable faces."
- ―David Bremner's description of Mason
- A Marriage Made In Heaven (Only if the bloody knife is found at the crime scene and returns to the bar later on )
- The Red Lipstick Murder
- The Golden Butterfly
- The White Shoe Slaying
- The Studio Secretary Murder
- The Quarter Moon Murders (Killed)
- Elizabeth Short - "The Black Dahlia"
- Celine Henry, The Bamba Club
- Deidre Moller, Belmont High School
- Antonia Maldonado, El Dorado Bar
- Theresa Taraldsen, Baron's Bar
- Evelyn Summers, Mensch's Bar
- Mason makes a brief optional appearance in A Marriage Made in Heaven. If the player finds the bloody knife on the first sweep of the scene and then returns to the bar later on, Mason will have replaced Dudley Lynch behind the bar, but he can't be interacted with.
- It is possible, but in no way confirmed, that the high-ranking politician in which Garrett Mason was related to was Lowell Blake Mason, who was appointed by President Harry S. Truman (President 1945-1953) to be the United States' Federal Trade Commissioner.
- It's possible to kill Mason very early in the chase in the church catacombs. Once you see Mason, you can chase after him and catch up to him. If you proceed to shoot him dead, it will end the chase early before it reaches the graveyard. However, by doing this, you miss most of Mason's dialogue during the chase.
- "I don't mean to be rude, but I've got work to do."
- "Unless you'd like a drink too, Officer, there's not a lot I can do for you."