The main part of Mosley's career has been 42 years as Librarian of the St. Bride Printing Library in London, where he curated and worked to expand the museum's large collection of printing and lettering materials, books and examples. This collection greatly expanded with the close of the metal type era, which saw many companies and printing shops selling off their equipment and archives. Mosley also expanded the library's collection of lettering and signs. He has also been a lecturer and professor at the University of Reading since 1964, and founded the British Printing Historical Society in that year.
Particular areas of focus of his career have been, in Britain, Eric Gill (with whose brother Evan he worked in the 1950s), William Caslon, Vincent Figgins and Talbot Baines Reed and, in Europe, the work of Claude Garamond and printing at the Imprimerie royale around the close of the 17th century.
Mosley grew up in Twickenham in south-west London, where he became interested in printing, before studying English at King's College, Cambridge, where he with Philip Gaskell, later also a historian of printing, operated a small hand-press as an amateur project in the college cellar. During his time at university he worked with Eric Gill's brother Evan on sorting material for an exhibition on his work by Monotype, a printing equipment company with which Gill often collaborated.
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.
Mosley helped to acquire for St. Bride a large range of printing materials, at a time when companies were disposing of their hot metal typesetting and foundry type equipment or going out of business altogether. This included material from Monotype, H. W. Caslon & Company, Figgins and the Chiswick Press, as well as materials from printing shops including the collections of Oxford University Press and the Victoria & Albert Museum, supplementing the personal collections of William Blades and Talbot Baines Reed which the library already owned. He has also advised on revivals of historic typefaces and lettering, for example one of traditional French metal stencil lettering.
Since retirement from St. Bride Mosley has continued to write, research and lecture, for example on the career of Eric Gill in 2015. He also advised on creating historically accurate lettering for replica globes, Tate Britain and HMS Victory.
- Barnes, Paul. "James Mosley: a life in objects". Eye magazine. Retrieved 11 December 2015.
- Mosley, James. "2003 Individual Award: acceptance speech". APHA. Retrieved 12 December 2015.
- Howes, Justin. James Mosley. Mark Batty. ISBN 0972424059.
- Mosley, compiled by Steven Tuohy ; with two essays by James (1995). James Mosley: librarian, St Bride Printing Library, London : a checklist of the published writings 1958-95. Cambridge: Rampart Lions Press. ISBN 9780902591608.
- Kinross, Robin. "Temple of Type". Eye. Retrieved 12 December 2015.
- "Ornamented types: a prospectus" (PDF). imimprimit. Archived from the original (PDF) on 22 December 2015. Retrieved 12 December 2015.
- Young, Timothy. "London Dispatch: The St. Bride Foundation". Design Observer. Retrieved 12 December 2015.
- "Professor James Mosley". University of Reading. Retrieved 11 December 2015.
- "James Mosley: Hyphen Press". Hyphen Press. Retrieved 11 December 2015.
- Mosley, James (2006). "Garamond, Griffo and Others: The Price of Celebrity". Bibiologia. Retrieved 3 December 2015.
- Vervliet, Hendrik D.L. (2008). The palaeotypography of the French Renaissance. Selected papers on sixteenth-century typefaces. 2 vols. Leiden: Koninklijke Brill NV. ISBN 9789004169821.
- Mosley, James. "Talbot Baines Reed, typefounder and sailor". Type Foundry (blog). Retrieved 12 December 2015.
- Barker, Nicolas. "Philip Gaskell: obituary". The Guardian. Retrieved 12 December 2015.
- Mosley, James (2001). "Memories of an Apprentice Typefounder". Matrix. 21: 1–13.
- "Eric Gill: Monotype Recorder special issue" (PDF). Monotype Recorder. 41 (3). 1958. Retrieved 6 November 2015.
- Richardson, Bob. "The Mosley effect". St. Bride Foundation. Retrieved 12 December 2015.
- "Change at Oxford...and at St. Bride's". Motif (1): 82.
- Mosley, James (1963). "English Vernacular". Motif. 11: 3–56.
- Mosley, James. "English vernacular (2006)". Type Foundry (blog). Retrieved 21 October 2017.
- Mosley, James. "The Nymph and the Grot: an update". Typefoundry blog. Retrieved 12 December 2015.
- Walters, John. Fifty Typefaces That Changed the World: Design Museum Fifty. Hachette. ISBN 1840916494.
- "Motif Magazine: the world made visible". Design Observer. Retrieved 12 December 2015.
- Kinross, Robin. "Justin Howes obituary". The Guardian. Retrieved 12 December 2015.
- Mosley, James (1999). The nymph and the grot: the revival of the sanserif letter. London: Friends of the St Bride Printing Library. ISBN 9780953520107.
- Norton, David (1988). "John Wilson, Hume's First Printer" (PDF). The British Library Journal. 14 (2): 123–135. Retrieved 12 December 2015.
- Mosley, James. "Handmade Type: Thoughts on the preservation of typographic materials". Incline Press. Retrieved 29 January 2016.
- Drucker, Margaret Re ; essays by Johanna; Mosley, James (2003). Typographically speaking : the art of Matthew Carter (2. ed.). New York: Princeton Architectural. ISBN 9781568984278.
- Mosley, James. "The materials of typefounding". Type Foundry. Retrieved 14 August 2015.
- Danielli, Darryl. "Interview: 'St Bride's is a living, breathing thing that pulls you in'". Print Week. Retrieved 12 December 2015.
- Mosley, James. "Lettres à jour: public stencil lettering in France". Type Foundry (blog). Retrieved 12 December 2015.
- Mosley, James (November 10, 2015). Lecture on Gill's work (Speech). 'Me & Mr Gill' talk. Old Truman Brewery, London.
- "HMS Victory to be re-painted in Battle of Trafalgar colours after 210 years". Portsmouth Historic Dockyard. Retrieved 12 December 2015.
- "Font Victory". Whybrow Wayfinding. Retrieved 12 December 2015.
- Mosley, James. "A British National Letter". Typefoundry (blog). Retrieved 12 December 2015.
- Foyle, Jonathan. "Globemaker Peter Bellerby, the man with the world in his hands". Financial Times. Nikkei. Retrieved 21 February 2016.