By 1938, Cassidy had earned his doctoral degree at the University of Michigan, and had married Hélène Lucille Monod, a fellow student.
Cassidy started as Chief Editor of the Dictionary of American Regional English in 1962, and saw Volume I (covering A-C) published by Harvard University Press in 1985.
Volume VI, subtitled "Contrastive Maps, Index to Entry Labels, Questionnaire, and Fieldwork Data," was released in 2013.
In 1967 he edited the Dictionary of Jamaican English, co-edited with Robert B. LePage, which drew from four centuries of written and oral usage.
The Cassidy System was later adopted and modified by the Jamaican Language Unit (JLU) at University of the West Indies,[7] and is now known as the Cassidy/JLU orthography.
In 2012 the Bible Society, in collaboration with JLU, translated the New Testament into Jamaican using the Cassidy orthography, it was published as Di Jamiekan Nyuu Testiment.