Alan Reiach was born in London on 2 March 1910, the son of Herbert L Reiach, a wealthy printer, amateur naval architect, and founding editor of Yachting Monthly, and Marie Barbara Federson who was Polish. He was brought up in Acton until the age of six and thereafter by his grandmother and aunt in Mill Hill until he was old enough to be sent to a prep school in Eastbourne. In 1921 his aunt moved to Edinburgh where he attended Edinburgh Academy from 1922 until 1928 when he was articled to Lorimer & Matthew: there he became a close friend of Robert Matthew who was four years older. In 1929 he made a study tour to East Anglia and to Wiltshire and Somerset the following year.

Whilst with Lorimer & Matthew he studied part-time at Edinburgh College of Art under John Begg, and he continued to study there for two years full-time after completing his apprenticeship in 1932. He obtained his diploma in June 1934, passed the Professional Practice exam the following month, and was admitted ARIBA on 3 December 1934, his proposers being Begg, John Fraser Matthew and Frank Charles Mears. During his final two years of study he won the Soane Medallion, the Tite Prize, the RIBA Silver Medal and an Andrew Grant travelling scholarship. The last of these was deferred a year while he took the postgraduate planning course, but in 1935-6 he visited France, Scandinavia, the USA (where he joined Frank Lloyd Wright for a short period at Taliesin) and the USSR. Like Robert Scott Morton and the slightly older Esmé Gordon he was a superb draughtsman and the sketches from his study tours are treasured by those fortunate enough to have them. On his return he spent about a year in London with Robert Atkinson and George Grey Wornum prior to becoming a research and teaching fellow at Edinburgh College of Art in 1938.

Reiach commenced practice in 1933 from his house in Randolph Place before completing his course at the College, his first commission being a house in Suffolk. In the years 1935 to 1937 it necessarily went into abeyance, but from 1938 he entered competitions with Robert Matthew, winning that for the baths and fire station at Ilkeston in 1938. He was also active in the Saltire Society, publishing a slim but influential volume entitled 'Building Scotland' with Robert Hurd in 1938.

In 1940 Robert Matthew brought Reiach into the Scottish Office as assistant secretary of the Scottish Housing Advisory Committee at the Department of Health. In the same year Reiach married Julie Dittmar, but the marriage did not survive wartime conditions in Scotland. His role at the Scottish Office ended in 1946 when he recommenced practice. With the help of a senior lecturing post at Edinburgh College of Art he managed to re-establish his practice in 1948, forming a single-project partnership with a colleague at the College, Ralph Cowan, to design the new College of Agriculture at the University of Edinburgh's King's Buildings site. In parallel he married a second time to Patricia Ann Duncan in 1949 and attracted an exceptionally able staff, George McNab in 1955 and Stuart Renton in 1956. By 1957 Reiach was so busy he had to resign from the College of Art and in the same year he took Renton into partnership, followed by McNab in 1958; and in the latter year John R Oberlander and Leslie D Mitchell joined the staff. Reiach was elevated to FRIBA in early 1959, his proposers being William Hardie Kininmonth, J Holt and Robert Hogg Matthew.

Reiach was a vice-president of the Edinburgh Architectural Association in 1959.

Late in 1964, Eric Hall proposed a merger with Reiach’s practice. The main objective being to achieve a scale and range of specialisations which would enable them to compete with the English practices then setting up offices in Edinburgh and Glasgow, a proposed duly implemented in 1965 when Reiach and his partners   moved into hall’s recently acquired office at 15 Moray Place. The practice title was then Alan Reiach, Eric Hall & Partners, subsequently abridged to Reiach & Hall. Although the senior partners were very different in temperament, Hall being calm and assured and Reiach somewhat mercurial. The partnership was extremely successful combining Hall’s management and technological expertise with the Reiach team’s design and presentation skills. The partnership now included Peter Caird Spence’s office and from 1967 John (Jack) Oberlander and Leslie D. Mitchell.

Reiach retired in 1975 but remained with the practice as part-time consultant until 1980. Because of the war years, his teaching commitments and the early involvement of partners in his university hospital and school projects, the period in which Reiach's own ability as an architect can be clearly recognised was short. In 2009, Stuart Renton wrote a vivid memoir of working with Alan Reiach, describing him as an intuitive designer who could turn a complex brief into a working concept remarkably quickly… he would bury himself in his studio at home until the small hours; appearing in the office with ‘muzzy but beautifully drawn’ sketch proposals which his partners had to develop into instructional realities.’ Bouncing ideas back and forth with Alan… These could be stormy times and anger might erupt… sometimes in exasperation he would disappear from the office, leap into his rather decrepit convertible and drive, perhaps more quickly than was wise, around the quiet New Town streets for an hour or so... When he reappeared all was fine and he left the project to be developed by his partners ‘intervening remarkably little.’ Reiach was appointed OBE in 1964, served on the Royal Fine Art Commission from 1966 until 1980 and was elected ARSA in 1969. He was elected full academician in 1986, but by that date his elevation owed more to his exceptional skill as draughtsman and watercolourist than to architecture.

In person Reiach was small in stature but very good-looking, always stylishly dressed with a bow tie and often a black beret which gave him a mildly Frenchified appearance. His personality was hugely engaging, witty, excitable and enthusiastic in a carefully controlled way, and intellectually challenging: he was always great fun to be with. He died on 23 July 1992 after a short illness. He was survived by his wife Patricia, one son and one daughter.