Harmon was born in Burbank, California, the youngest of three children. His parents were Heisman Trophy winning-football player and broadcaster Tom Harmon and actress and artist Elyse Knox (née Elsie Lillian Kornbrath). Harmon has two older sisters, actress and painter Kristin Nelson, the former wife of singer Ricky Nelson, and actress-model Kelly Harmon, who was once married to car magnate John DeLorean. His maternal grandparents were Austrian immigrants.

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