Starting his career at
Wellesley College as an instructor in botany, Riddle later became a full professor in 1917. During this tenure, he managed the lichen
herbarium previously overseen by
Clara Eaton Cummings. His proximity and access to both this collection and the remarkable one at Harvard bolstered his reputation among American and European lichenologists. In 1913, Riddle took a year's leave from Wellesley to study lichen collections and collaborate with botanists in various European cities including
London,
Paris,
Geneva,
Uppsala and
Helsingfors. He returned to the U.S. and, in 1919, was appointed assistant professor of Cryptogamic Botany and Associate Curator of the Cryptogamic Herbarium at Harvard. Riddle was actively involved in the New England Botanical Club, serving as its Cryptogamic Curator from 1910 to 1917, and its president from 1917 to 1920. Before falling ill, he was designated as an associate editor of the scientific journal
Rhodora, succeeding the late
Frank Shipley Collins. He also held the role of associate editor for
The Bryologist from 1911. Furthermore, he was a Fellow of the
American Academy of Arts and Sciences and a member of the
Botanical Society of America. His publications predominantly centered on systematic lichenology. In his final published work on
Acrospermum, Riddle began exploring other areas of mycology. ==Death==