Hamm is of 37.5% German, 37.5% English, and 25% Irish descent.
Jon Hamm was born on March 10, 1967, in St. Louis, Missouri, the United States, Deborah (Garner) and Daniel Hamm. His father had two daughters from his first marriage. Hamm also has two siblings. An older sister named Jennifer Hinze and a younger sister, Julie Schulte.
Hamm had a difficult upbringing. His family owned a successful trucking company for generations in St. Louis, but the business declined as Jon grew up. When Hamm was just two, his parents got divorced. He lived with his mother in St. Louis County in Creve Coeur, Missouri. He saw his father on the weekends. Living with his mother, he "tried a lot of different things - violin, soccer, creative writing," Hamm told in an interview "My mother celebrated learning." Subsequently, Hamm went to live with his father in Clayton, Missouri.
At the age of 10, Hamm lost his mother to colon cancer, which had spread throughout her body. After her death, Hamm moved to his grandmother's house with his father. He recalled: "We were three generations living under one roof, which is difficult in the best of circumstances." Some of his friends' mothers also felt moved to look after him. His father passed away when he was 20.