After a brief period working at the type foundry Stevens Shanks, one of the last remaining in London, Mosley was hired at St. Bride as assistant librarian in 1956, becoming librarian in 1958. As a writer, two of his most famous articles are 'English Vernacular', on signpainting and lettering traditions, and 'The Nymph and the Grot', on the early development of sans-serif letters before they became adopted by printers, which was later republished as a book. He has collaborated with historians on other projects, for example on a study of the early printing of works by Hume and with Justin Howes. He also worked with Harry Carter, and has also contributed to a book on his son Matthew.
