Thomas Riddell's research and teaching interests previously focused on U.S. and global public policy, particularly the economics of military spending and the environmental dimensions of global economic activity. More recently, he has taught a first-year seminar on The Evolution and Transformation of the Northampton State Hospital and has been contributing to efforts to memorialize the history of the hospital.

He is the co­-author of a one­-semester introductory economics textbook in its ninth and final edition. From 1993 through 2011, Riddell was the dean of the first-year class and the associate dean of the college from 2000 through 2011.