Leonard Allinson was born 1 May 1926, the only son of Walter Allinson and Alice Frances Cassidy; in 1951 he married Margaret Patricia Watts. He was educated at Friern Barnet Grammar School and Merton College, Oxford, taking a First class in History in 1947.
During his career, Allinson served as: Assistant Principal, Ministry of Fuel and Power (Petroleum Division), 1947-48; Assistant Principal, later Principal, Ministry of Education, 1948-58 (Assistant Private Secretary to Minister, 1953-54). He transferred to the CRO in 1958 and thereafter became: First Secretary in Lahore and Karachi [Pakistan], 1960-62, Madras and New Delhi [India], 1963-66; Counsellor and Head of Political Affairs Department, March 1968; Deputy Head, later Head, of the Permanent Under Secretary's Department, FCO, 1968-70; Counsellor and Head of Chancery, subsequently Deputy High Commissioner, Nairobi [Kenya], 1970-73; RCDS, 1974; Diplomatic Service Inspectorate, 1975; Deputy High Commissioner and Minister, New Delhi, 1975-77; High Commissioner, Lusaka [Zambia], 1978-80; Assistant Under-Secretary of State (Africa), 1980-82; High Commissioner in Kenya and Ambassador to UN Environment Programme, 1982-86.
He was awarded the KCVO in 1979 (MVO 1961); CMG 1976.