Colin Edgar McWilliam was born in 1928 in Woolwich, London to Ormiston Galloway Edgar McWilliam (1893-1942) and his wife Ida Margaret Bradbury (d.1972), who had married in East Grinstead in 1923. His father was a Flight Lieutenant in the RAF and died when his plane crashed after returning from a raid.
Colin graduated in Architecture at Cambridge and after a spell at the British School in Rome moved to Scotland in late 1951 to become a draughtsman for the Scottish National Buildings Record. Around this time he assisted the architect Stewart Kaye and also practised briefly as an architect with Ian Hodkinson. In 1957 he became Assistant Secretary of the National Trust for Scotland.
From 1964 he taught History of Architecture at Heriot Watt University, based at the Edinburgh College of Art, becoming the senior lecturer and then Director of the postgraduate Conservation course. He was chairman of the Scottish Georgian Society (later Architectural Heritage Society for Scotland) and battled alongside Eleanor Robertson against the University's redevelopment proposals for George Square and the southside.
McWilliam also served as a member of the Historic Buildings Council for Scotland and the New Town Conservation Committee.
He is known for his authorship of several books including Scottish Townscape (1975) and the Buildings of Scotland volumes for Lothian (1978) and as the co-author for Edinburgh (1984). He was also the architecture correspondent for the Scotsman Newspaper.
He was an Hon. member of the RIBA and RIAS.
He died in Edinburgh on 8th December 1989 aged 61.