Todd was born near Glasgow, attended Allan Glen's School and graduated from the University of Glasgow with a B.Sc. in 1928. He received a Ph.D (Dr.rer.nat.) from Johann Wolfgang Goethe University of Frankfurt am Main in 1931 for his thesis on the chemistry of the bile acids. After studying at Oriel College, Oxford he gained another doctorate in (1933) and held posts with the Lister Institute of Preventive Medicine, London, and the University of London.
Todd became the Sir Samuel Hall Professor of Chemistry and Director of the Chemical Laboratories of the University of Manchester in 1938, where he began working on nucleosides, compounds that form the structural units of nucleic acids (DNA and RNA). In 1949 he synthesized adenosine triphosphate (ATP) and flavin adenine dinucleotide (FAD).
In 1955 he elucidated the structure of vitamin B12, later working on the structure and synthesis of vitamin B1 and vitamin E, the anthocyanins (the pigments of flowers and fruits) from insects (aphids, beetles) and studied alkaloidss found in hashish and marijuana. He served as chairman of the British government's advisory committee on scientific policy from 1952 to 1964.
He was professor of organic chemistry and Fellow of Christ's College, Cambridge (1944-1978, was Master of Christ's College (1963-1978) and became Chancellor of the University of Strathclyde in 1975, a member of the Royal Order of Merit in 1977 and a visiting professor at Hatfield Polytechnic (1978-1986).
Lord Todd was married to Alison Sarah, daughter of Nobel Prize winner Sir Henry Dale, and had a son, Alexander Henry, and two daughters, Helen Jean and Hilary Alison.