He was the founding dean of the School of Social Welfare at University of California, Berkeley in the early 1940s before resigning to work for the United Nations Relief and Rehabilitation Administration.
He went on to hold distinguished posts in public service, and in the 1930s and 1940s contributed to national debates about the role of the state in welfare and education.
His research influenced Bennett's decision to try to emulate Franklin Delano Roosevelt's New Deal and some of his ideas were ultimately taken up by the Liberal government of William Lyon Mackenzie King in the 1940s, particularly after Cassidy published his 1943 study Social Security and Reconstruction in Canada.
He continued to criticize Canada's social security system as inadequate, writing in 1947 that "The provisions for general assistance are limited, restrictive, mean, and antiquated ...
After Cassidy joined the army he began to keep a diary, perhaps inspired by recognition of a change in his social role and its familiar routines.