In 1960 Souter received the Walkley National Award for Australian Journalism.
His books have included New Guinea: The Last Unknown (1963), a history of the exploration of all the territories of New Guinea, written after he had accompanied an initial penetration patrol making first contact with the Gants people of the Jimi Valley; A Peculiar People (1968), an account of the New Australia Utopian settlement in Paraguay; The Idle Hill of Summer: an Australian Childhood 1939-1945 (1972); Lion & Kangaroo: The Initiation of Australia 1901-1919 (1976). In 1981 he was commissioned by John Clunies-Ross, "King" of the Cocos (Keeling) Islands, to research and write the history of that principality in the Indian Ocean. After completion, the manuscript was not published as the Commissioner felt that it differed from his own recollection of events during his rule.
In 1983 the Federal Parliamentary Library commissioned him to write the narrative history of the Parliament of Australia. Acts of Parliament was published in 1988 as part of the Parliament's commemoration of the Bicentennial of European settlement in Australia.
Among other books by Souter are two histories of John Fairfax Limited: Company of Heralds (1981), which won the Foundation for Australian Literary Studies award;[3] and Heralds and Angels: the House of Fairfax 1841-1990 (1992), which won the Victorian Premier's Literary Award.[3]
He was Vice President of the Australian Society of Authors 1975–78, Deputy Chairman of the Commonwealth Films Board of Review 1981–84, and is Patron of Mosman Historical Society.
His wife of 62 years, Ngaire (née Avison), died on 26 July 2014. He has two daughters, Anne and Gretel, and one granddaughter, Rachel Blackburn.
Company of Heralds: a Century and a Half of Australian Publishing by John Fairfax Limited and its Predecessors 1831-1981, Melbourne University Press, 1981. ISBN0-522-84218-6
Acts of Parliament: A narrative history of Australia's Federal Legislature, Melbourne University Press, 1988. ISBN0-522-84408-1
Heralds and Angels: the House of Fairfax 1841-1990, Melbourne University Press, 1991. ISBN0-522-84449-9