King was born in Wellington to Eleanor and Commander Lewis King, one of four children. Educated at Sacred Heart College in Auckland and St. Patrick's College, Silverstream, he went on to study history at Victoria University of Wellington before working as a journalist at the Waikato Times newspaper in Hamilton in 1968.
King earned degrees in history at Victoria, (BA 1967) and the University of Waikato (MA 1968), and gained his PhD at Waikato (1978). In 1997 he received an honorary DLitt at Victoria. He was Visiting Professor of New Zealand Studies at Georgetown University in Washington, D.C., and taught or held fellowships at six other universities.
Although not Māori himself, King was well known for his knowledge of Māori culture and history. New Zealand Listener, one of New Zealand's most popular weekly magazines, dubbed King "the people's historian" for his efforts to write about and for the local populace. As a biographer, King published works on Te Puea Herangi, Whina Cooper, Frank Sargeson (1995) and Janet Frame (2000). As an historian, King's works include Being Pākehā (1985), Moriori (1989), and The Penguin History of New Zealand (July 2003), the latter of which was, by February 2004, into its seventh edition. In all, King wrote, co-wrote and edited more than 30 books on a diverse range of New Zealand topics. He contributed to all five volumes of the Dictionary of New Zealand Biography.
King was always sensitive to the fact that he was a Pākehā writing about the Māori world and always sought to establish close personal relationships with those he wrote about and their whānau, hapū and iwi authorities. He believed that all Pākehā had the same right to be called indigenous as Māori and disagreed with claims that only Māori have a spiritual association with mountains, lakes and rivers. He noted a recent tendency in literature to romanticise Māori life and indicated that certain aspects of Māori society in the pre-European era was harsher and less humane than the results of British colonisation.
King was a diabetic and had post-polio syndrome. He received six weeks of chemotherapy and radiotherapy for throat cancer discovered in October 2003, which was in remission by 2004. In 1984 he was living with his family in a commune. His wife Ros decided to live with someone else and eventually married them.
Following King's death, an essay on John Money was posthumously published in an exhibition catalogue for the Eastern Southland Gallery, located in the provincial town of Gore, New Zealand. King had planned to write a full biography on Money, but had lacked funding to do so in his lifetime.
He has two children, the filmmaker Jonathan King and novelist Rachael King.
King and his second wife, Maria Jungowska, were killed when their car crashed into a tree and caught fire near Maramarua, on State Highway 2 in the north Waikato. The cause of the crash was reported by the police at the time to be a complete mystery since speed was not a factor and investigators have little idea why the car would veer off a straight road.
A coroner's inquest into the deaths determined that the accident was most likely caused by driver inattention.
King was winner of the 2003 Prime Minister's Award for Literary Achievement in Non-Fiction. Throughout his career he won the Feltex Television Writers' Award (1980), Winston Churchill Fellowship (1980), Fulbright Visiting Writers' Fellowship (1988), Order of the British Empire (1988), NZ Literary Fund Award (1987 and 1989), Wattie Book of the Year Award (1984 and 1990), NZ Book Award (non-fiction) (1978) and was Burns Fellow at the University of Otago (1998–99). His book The Penguin History of New Zealand was overwhelmingly the Readers' Choice at the 2004 Montana NZ Book Awards. The New Zealand Herald named him New Zealander of the Year for 2003.