His family owned a successful trucking company for generations in St. Louis, but the business declined as Jon grew up. His parents divorced when he was only 2 years old. Living with his mother, Deborah, he "tried a lot of different things—violin, soccer, creative writing".
Hamm also became very active in the university's theater program. One of his professors, Jim Miller, said that Hamm "was the most intellectual actor I have had in 30 years of teaching," according to an article in the university's alumni magazine, Mizzou. He spent two summers performing in the school's repertory company, starring as Cliff in Cabaret and Leon Colgosz in a production of Stephen Sondheim's musical Assassins.
In 1995, Hamm decided to move to Los Angeles to pursue an acting career. Packing up everything he owned, he drove to his aunt and uncle's house in West Los Angeles. He stayed with them until he got his own apartment, which he shared with a series of roommates. For years, Hamm struggled to find work as an actor. He made his television and film debuts in 2000, with bit parts in the comedy series The Hughleys and the adventure flick Space Cowboys.
In 2007, Hamm took on his most famous character to date, playing philandering ad executive Don Draper in the American Movie Classics show Mad Men. He auditioned seven times to get the role.
In 2014, Hamm starred in the heartwarming sports movie The Million Dollar Arm. The film is based on a true story about a sports agent who recruits several Indian cricket players and tries to transform them into major league baseball pitchers.