Born on 5 December 1907 at Wick, William Barclay was the son of William Dougal Barclay, bank manager, and of Barbara Linton McLeish. He was educated at Dalziel High School, Motherwell, where he became Captain of the School, Proxime Accessit to Dux of the School, and Sports Champion.
He proceeded to Glasgow University where he graduated M.A. in 1925 with First Class Honours in Classics, besides being prizeman in English Political Economy and Latin. In 1932 he graduated B.D. with distinction in New Testament Studies, besides being Maxwell Forsyth Fellow and winning the Dods New Testament Prize and the Dykes Practical Training Prize. From 1932 till 1933 he carried out post graduate study at Marburg University.
He was Student Assistant Minister at Newlands South Church, Glasgow, from 1931 -1932, and was licensed by the Presbytery of Glasgow on 17 March 1932.
On 22 February 1933 he was ordained and inducted to Renfrew Trinity Church, where he remained until he demitted office on 31 December 1946 on taking up his appointment as Lecturer in New Testament Language and Literature at Glasgow University. He was later appointed to the position of Senior Lecturer in New Testament and Hellenistic Greek.
In 1956, Barclay’s scholarship was recognised by Edinburgh University, which conferred on him the Degree of Doctor of Divinity.
Professor Barclay was also the Examiner in New Testament at the Universities of Aberdeen, Edinburgh, St. Andrews and Leeds.
He held the Bruce Lectureship (1936 – The Use of the New Testament in the Early Fathers), The Kerr Lectureship (1954-1957 – Educational Ideals in the Ancient World), and the Croall Lectureship (1954-1957 – The Ethical Vocabulary of Saint Paul).
Other academic positions held by Professor Barclay were membership f the Society for New Testament Studies and of the International Conference on Patristic Studies. He was also a director of the National Bible Society of Scotland, a member of the Translating Committee of the New English Bible, and translator on the Apocryphal Panel.
He regularly lectured in various parts of Great Britain and conducted courses for Methodist Lay Preachers, Anglicans in Wycliffe Hall, Oxford, the Royal Army Chaplains’ Department, as well as Public Lectures for the University of Wales at Bangor University College.
From 1943 until 1947 Barclay was Editor of the Sunday School Teachers Magazine, and contributed a regular weekly page in the British Weekly, a regular monthly page in the Expository Times and was a regular contributor to Young Scotland. He was also a well-known and popular broadcaster.
On 30 June 1933 he married Katherine Barbara Gillespie, daughter of the Rev James Hogg Gillespie of Dundonald. They had three children, James William Ronald, M.A., Barbara Mary, who tragically drowned at sea on 11 August 1956, and Jane Anne.
In the New Year Honours List of 1969 he was made a Commander of the British Empire
Professor Barclay died in his sleep in the early hours of Tuesday 24 January 1978.