Debnam's early years of schooling were at Frenchs Forest Public School and The Forest High School. Debnam was educated at the Royal Australian Naval College, where he graduated in 1974. He served in the Royal Australian Navy from 1972 to 1980. During his Naval career, Debnam served on many ships including the aircraft carrier , destroyer escort , destroyers and and patrol boats and . After leaving the Navy, Debnam studied at the Macquarie Graduate School of Management, where he gained an MBA. He held positions at Dalgety Farmers Limited, Hawker de Havilland and Australian Aircraft Consortium before entering politics.