Thomas Griffin Dunne was born on June 8, 1955, in New York City, to Ellen Beatriz and Dominick Dunne. Growing up in Los Angeles alongside his siblings Alexander and Dominique, he was immersed in a world of storytelling and creativity. His father was a prominent figure in the entertainment industry, working as a producer, writer, and actor, while his mother later founded the victims' rights organization Justice for Homicide Victims after Dominique's tragic murder in 1982.
From a young age, Dunne showed a passion for acting, which led him to attend the Fay School in Southborough, Massachusetts, and later the Fountain Valley School in Colorado Springs. It was during these formative years that Dunne's talent for acting began to shine, as he participated in numerous school plays and developed his craft.