Of Ngāti Tūwharetoa descent, King was born in Whanganui and grew up in Turangi, where his father was a prison officer. He was head prefect at Tongariro High School in 1987. The following year, he represented New Zealand as one of 32 pavilion hosts in the World Expo in Brisbane, Australia. King studied law at the University of Otago, graduating (LLB) in 1992. He was admitted to the bar on 14 May 1993 at Dunedin High Court.