He was born in Sydney and educated at Kempsey High School, and Scots College in Warwick, Queensland and then graduated BA from the University of Sydney. He joined The Sydney Morning Herald as a journalist in 1947 and worked there for the next 40 years, serving as a correspondent in New York and London  and later as an Assistant Editor of The Herald.
In 1960 Souter was awarded the W.G. Walkley National Award for Australian Journalism.
His books have included New Guinea: The Last Unknown (1963); A Peculiar People (1968), an account of the New Australia settlement in Paraguay; The Idle Hill of Summer: an Australian Childhood (1972); Lion & Kangaroo: The Initiation of Australia 1901-1919 (1976).
In 1983 he was commissioned to write a narrative history of the Parliament of Australia to commemorate the Bicentenary of European occupation of Australia in 1988. This was entitled Acts of Parliament.
He also wrote two histories of John Fairfax Limited - Company of Heralds (1981), which won the Foundation for Australian Literary Studies award; and Heralds and Angels: the House of Fairfax 1841-1990 (1992), which won the Victorian Premier's Literary Award.
He was Vice President of the Australian Society of Authors 1975-78 and Deputy Chairman of the Commonwealth Films Board of Review 1981-84.
List of works
- New Guinea: the Last Unknown. Sydney. Angus and Robertson, 1963. ISBN 0-207-94627-2
- "Sydney Observed, Sydney, Angus & Robertson, 1968 Cat. Card No. 62-25528 Registry No. 68-1817
- A Peculiar People: the Australians in Paraguay. Sydney, Angus and Robertson, 1968. ISBN 0-207-95037-7
- The Idle Hill of Summer: an Australian Childhood, 1939-1945. Sydney, Angus and Robertson, 1972. ISBN 0-207-12370-5
- Lion and Kangaroo: the Initiation of Australia, 1901-1919. Sydney, Collins, 1976. ISBN 0-00-211445-3
- Company of Heralds: a Century and a Half of Australian Publishing by John Fairfax Limited and its Predecessors, 1831-1981. Carlton, Vic. Melbourne University Press, 1981. ISBN 0-522-84218-6
- Acts of Parliament: a Narrative History of the Senate and House of Representatives, Commonwealth of Australia. Carlton, Vic., Melbourne University Press, 1988. ISBN 0-522-84408-1
- Heralds and Angels: the House of Fairfax 1841-1990. Carlton, Vic. Melbourne University Press, 1991. ISBN 0-522-84449-9
- Mosman: a History. Carlton, Vic., Melbourne University Press, 1994. ISBN 0-522-84591-6
- A Torrent of Words: Leon Gellert: a Writer's Life. Canberra, Brindabella Press, 1996. ISBN 0-909422-26-5
- Times & Tides: a Middle Harbour Memoir. Pymble, N.S.W. Simon & Schuster, 2004. ISBN 0-7318-1224-7
- (Unpublished MS, held by the National Library of Australia: Cocos. A History of the Clunies-Ross Family. Commissioned by John Clunies-Ross)