For his PhD, he moved to Sheffield to study ecology under David Read, researching the effect of sulphur dioxide pollution on pasture grasses.
Crittenden started working with lichens after moving to Canada as a research fellow at McMaster University.
Crittenden eventually settled into a lectureship at the University of Nottingham in 1981,[4] where he remains and is Leverhulme Emeritus Professor as of 2021.
[6] Crittenden implemented several important editorial changes, including the adoption of effective electronic publication, obligate registration of new fungal names, and a policy to reject "single naked species descriptions" from 2016 onwards.
[6] Crittenden collaborated with James D. Lawrey, editor of The Bryologist, to increase the quality of both journals, further enhancing the impact of lichenological research.