Fagg spent a considerable amount of time engaged in fieldwork in Africa: the Belgian Congo in 1949–1950; Nigeria in 1953, 1958–1959, 1971, and 1981; Cameroon in 1966; and Mali in 1969.
In 1956 he was replaced as the Royal Anthropological Institute's General Secretary by the American anthropologist Marian Smith.
[2] William Fagg purchased Benin art for the newly founded Lagos museum during his 1958–1959 trip to Nigeria.
He donated his photographic negatives and related documentation to the Royal Anthropological Institute shortly before his death so they could be used for research purposes by others.
It also led him to write the book 'Nigerian Images' (1963) for which he won the Amaury Talbot Prize for African Anthropology.