Iain Coutts MacWilliam was born in Port Glasgow on 20 May 1926. He attended Chapelton School in the town, and then Port Glasgow High School, Macduff High School, and Trinity Academy in Edinburgh. Edinburgh University followed, with matriculation in October 1943. He graduated with the degree of B.Sc. in January 1946, and was awarded a 2nd class Honours degree in Chemistry in July 1948, before a Ph.D. (Science) in December 1950. Later on, in October 1968, he was awarded the degree of D.Sc. for his thesis entitled Carbohydrate and other studies related to fermentation.

After studies, between 1946 and 1947, MacWilliam's career took him to A.E.L., the Chemical Research Laboratory (Department of Scientific and Industrial Research) at Teddington, Middlesex, then, in the 1950s to the Brewing Research Foundation. In the 1960s and 1970s at the Brewing Research Foundation, he was a senior then principal scientist, first in the Analytical Section, then Malting Section, then Biochemical Section. He retired from the Foundation in 1986, though continued to do some work with its Library and Archives Section.

Dr. Iain Coutts MacWilliam became a member of the Edinburgh University Club of London in 1946. He was elected as a Fellow of the Royal Institute of Chemistry in 1969, and a Fellow of the Institute of Brewing in 1988.

Dr. Iain Coutts MacWilliam died in 2013.