Hamlin was born in Pasadena, Californa. He attended Flintridge Preparatory School, close to Pasadena, and The Hill School, a private boarding school in Pennsylvania. When he was a teenager he played soccer, and lacrosse, and he acted in the school musicals and plays.
After high school Hamlin went on to the University of California.
Hamlin has a son, Dimitri Alexandre Hamlin, born 1980, with the swiss actress Ursula Andress.
Hamline has been married three times. He was married to actress Laura Johnsson from 1985 to 1989, and to Nicolette Sheridan, now known for her role in Desperate Housewives, from 1991 to 1993.
Hamlin married the actress and television host Lisa Rinna in 1997. They have two children together, Delilah Belle (born 6/10/98) and Amelia Gray (born 6/13/01).