Widdowson was born in Wallington, Surrey. Her father was a grocer's assistant. Her younger sister Eva Crane trained as a nuclear physicist but became a world renowned authority on bees.
She lived in Dulwich and attended Sydenham County Grammar School for Girls. She studied chemistry at Imperial College, London and graduated with a BSc in 1928, becoming one of the first women graduates of Imperial College. She did postgraduate work at the Department of Plant Physiology at Imperial College, receiving a PhD in chemistry in 1931 for her thesis on the carbohydrate content of apples. She did further research with Professor Charles Dodds at the Courtauld Institute of Biochemistry at Middlesex Hospital, on the metabolism of the kidneys, and also received a doctorate from the Courtauld Institute.
Dodds suggested that Widdowson should consider specialising in dietetics, and she started a postgraduate diploma at King's College, London. Widdowson met Robert McCance in the kitchens at St Bartholomew's Hospital in 1933, where she was studying industrial cooking techniques as part of her diploma on dietetics. McCance was a junior doctor researching the chemical effects of cooking as part of his clinical research on the treatment of diabetes. Widdowson pointed out an error in McCance's analysis of the fructose content of fruit, and they both realised that there were significant errors in the standard nutritional tables. They became scientific partners and worked together for the next 60 years, until McCance died in 1993.
McCance became a Reader in Medicine at Cambridge University in 1938, and Widdowson joined his team at the Department of Experimental Medicine in Cambridge. They worked on the chemical composition of the human body, and on the nutritional value of different flours used to make bread. Widdowson also studied the impact of infant diet on human growth. They studied the differing effects from deficiencies of salt and of water, and produced the first tables to compare the different nutritional content of foods before and after cooking. Their work became of national importance during the Second World War. Widdowson and McCance were co-authors of The Chemical Composition of Foods, first published in 1940 by the Medical Research Council. Their book "McCance and Widdowson" became known as the dietician's bible and formed the basis for modern nutritional thinking.
Widdowson and McCance and their colleagues became their own experimental subjects, eating a starvation diet of bread, cabbage and potatoes for several months to find out if wartime rationing - with little meat, dairy or calcium intake - would affect their health. They showed that good health could be supported by a very restricted diet, with calcium supplements, and their work became the basis of the wartime austerity diet promoted by the Minister of Food Lord Woolton.
Widdowson and McCance headed the first mandated addition of vitamins and mineral to food. Their work began in the early 1940s, when calcium was added to bread. They were also responsible for formulating the war-time rationing of Britain during World War II.
Widdowson and McCance were employed by the Medical Research Council from 1946, and spent most of their working life in Cambridge. They were consulted on the rehabilitation of the victims of severe starvation in Nazi concentration camps, and visited Holland, Germany and Denmark in early 1946 to study of the impact of the poor wartime diet on the people in Nazi-occupied territories. Widdowson followed up this work in the 1950s, 1960s and 1970s by studying malnourishment in Africa. Research on animals showed that malnourishment in early life led to lifelong effects on growth and health.
Widdowson showed that a new-born human infant has 16 per cent of its weight as fat, much greater than the one or two per cent of other species. She also studied the importance of the nutritional content of infant diets, particularly trace vitamins and minerals in natural and artificial human milk. Her work led to revised standards for breast milk substitutes in the UK in the 1980s.
Widdowson became head of the Infant Nutrition Research Division at the Dunn Nutritional Laboratory in Cambridge in 1966. She formally retired in 1972, but continued academic research in the Department of Investigative Medicine at Addenbrooke's Hospital. She was president of the Nutrition Society from 1977 to 1980, president of the Neonatal Society from 1978 to 1981, and president of the British Nutrition Foundation from 1986 to 1996. She became a Fellow of Imperial College in 1994.
She became a Fellow of the Royal Society in 1976 and was appointed a CBE in 1979. She was made a member of the Order of the Companions of Honour in 1993, which is awarded for outstanding achievements in the arts, literature, music, science, politics, industry, or religion.
The British Nutrition Foundation published a book in 1993 to celebrate 60 years of her partnership with McCance, McCance & Widdowson. A Scientific Partnership of 60 Years, 1933–1993, edited by Margaret Ashwell.
Widdowson lived in Barrington near Cambridge for over 50 years. She ate a simple diet, including butter and eggs, and attributed her longevity to good genes: her father lived to 96 and her mother to 107. She died at Addenbrooke's Hospital after suffering a stroke while on holiday with her sister in Ireland. She never married.