Around 1910 Pearson began to research the larger fungi, encouraged by John Ramsbottom, mycologist at the Natural History Museum.

In 1948 he produced a checklist of British species, co-authored with Dr R. W. G. Dennis of the Royal Botanic Gardens, Kew.

Pearson spoke several European languages, collected fungi in Spain and Portugal (on which he wrote brief papers), was an active member of the Société mycologique de France, and was credited with introducing new, continental ideas on modern agaric taxonomy into Britain.

In 1948 he was invited to survey fungi in South Africa, publishing a number of new species as a result.

His last works were a series of keys to British agaric genera, several of which were published posthumously by his friend and fellow mycologist P. D. Orton.