Born in Arizona, Jahn spent much of his youth moving between his father's apartment in Austin, Texas, and his mother's various rentals in and around Los Angeles, California. At one point, while living near Los Angeles, he was one of six people sharing a one-bedroom apartment, and has said it was to avoid these cramped living quarters that he spent much of his time in public libraries. He left school at sixteen, and later joined the army, an experience he has described as "ludicrous." He now lives in Los Angeles with his wife Mary.

Though his work has been described as crime fiction, including by Jahn himself ("It revolved around a crime, so that seemed to be what it was"), he has cited writers as diverse as Raymond Carver, Ernest Hemingway, and Stephen King as influences.

His first novel, Acts of Violence, takes place over a period of three hours and revolves around the murder of Katrina Marino, a bar manager stabbed in the courtyard outside her apartment block. Her murder is witnessed by several people, none of whom call the police or try to help, as each denies responsibility, an example of the bystander effect. Though inspired by the murder of Catherine Genovese, the case that first spurred research into the diffusion of responsibility, Jahn has said he wasn't interested in writing a didactic work. "I didn't want to create symbols; I wanted the story to be inhabited by people."

Jahn's second novel, Low Life, was released on 2 July 2010. He has said in interview that "it’s about a payroll accountant who lives a lonely, friendless life. One night while he’s asleep in bed someone breaks into his apartment and tries to murder him. During the altercation he ends up killing his assailant, and rather than calling the police, he puts the body on ice in his bathtub and tries to find out who wanted him dead and why."