Born in Ulster, the son of a linen merchant, he was educated at St. Bees School, before wartime service in the Royal Naval Air Service, piloting an observation aircraft from a warship. After the war, he read natural sciences at Cambridge University and then went on to study medicine at King's College Hospital in London. He then began a scientific career in the study of nutrition.
With colleague H. Shipp, he published The Chemistry of Flesh Foods and their Losses on Cooking in 1933. He co-authored the long-standard text and reference book The Chemical Composition of Foods in 1940 with Elsie Widdowson, his science co-worker. Their work became known as the basis for modern Western nutritional thinking, with editions in print from 1940 to 2002. McCance and Widdowson played a leading part in wartime rationing and 1940s government nutrition efforts. In 1936, he delivered the Goulstonian Lecture to the Royal College of Physicians on the subject of Medical problems in mineral metabolism.
He was later the director of the Medical Research Council's infantile malnutrition unit in Kampala, Uganda. He was elected a Fellow of the Royal Society in 1948 and made CBE in 1953. He died on 5 March 1993. 
In 1993 the British Nutrition Foundation issued McCance & Widdowson – A Scientific Partnership of 60 years ( McCance and Widdowson: A Scientific Partnership of 60 Years, 1933-93 - A Commemorative Volume Prepared as a Tribute to the 60-year Scientific Partnership Between Robert Alexander McCance CBE, FRS and Elsie May Widdowson CBE, FRS Margaret Ashwell (Editor) ISBN 0-907667-07-4 )
- Widdowson, E. M. (1995). "Robert Alexander McCance. 9 December 1898-5 March 1993". Biographical Memoirs of Fellows of the Royal Society 41: 262–226. doi:10.1098/rsbm.1995.0016.
- Widdowson, EM (1995). "Robert Alexander McCance – 9 December 1898—5 March 1993". Biographical memoirs of fellows of the Royal Society. Royal Society (Great Britain) 41: 261–80. PMID 11615357.
- "The Robert McCance Lecture". Retrieved 2012-10-17.
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