He was the author and editor of several works on the archaeology of Roman Britain and the Germanic peoples in the Migration Period.

Encouraged by his father, who did not want his son to become a miner, he received his Bachelor of Arts (BA) degree in classics from St David's College, Lampeter, and a diploma in classical archaeology from Brasenose College, Oxford, in 1963.

[1] During his studies, Todd became strongly interested in the archaeology of the Rhine provinces of the Roman Empire, and from 1963 to 1965 he worked as a research assistant at Rheinisches Landesmuseum Bonn under Harald von Petrikovits.

He was the first archaeologist appointed a professor at Exeter since the teaching of archaeology had been established there some years earlier under Aileen Fox.

Todd uncovered a previously unrecognised Roman occupation of the Iron Age hillfort in Hembury.

Todd was editor of Britannia for five years, and was later chairman of its editorial committee, which also oversees the publishing of the Journal of Roman Studies.

[3][4] Todd was on the Royal Commission on the Historical Monuments of England from 1986 to 1992, and on the council of the National Trust from 1987 to 1991.

[3] Under his leadership, significant changes were made to the management structure of Trevelyan, new fellowships, scholarships and awards were introduced, and its buildings were renovated.

[7] Though well liked by many of his students, Todd had little patience with administrative work, and had an ambivalent view on the digitization of academia.

[3] Throughout his career, he was the author of numerous books and scholarly articles, and also wrote entries for works such as the Encyclopædia Britannica, Cambridge Ancient History and Oxford Dictionary of National Biography.