Professor Mullan is recognized as one of Canada's foremost scholars in Administrative Law. He lectured at the Victoria University of Wellington before joining the Queen's University Faculty of Law in 1971. Apart from four years (1973–1977) at Dalhousie Law School, he remained at Queen's until January 2004. Professor Mullan has taught a range of private and public law courses, but his major area of academic interest has been administrative law, a field in which he has written extensively. He is co-author of Administrative Law: Cases, Text and Materials, now in its fourth edition, written in collaboration with faculty members at Osgoode Hall Law School and the University of Toronto. He is also the author of the Administrative Law Title in the Canadian Encyclopedia Digest. Over the past 25 years, he has served as consultant on a number of law reform projects and is currently a panelist under Chapter 19 of the North American Free Trade Agreement and a part-time member of the Ontario Human Rights Code Board of Inquiry. Professor Mullan has been a recipient of the Queen's University Alumni Award for Excellence in Teaching, the Queen's University Prize for Excellence in Research, and in 1996, the Canadian Association of Law Teachers` Award for Academic Excellence.
In July 2004, David Mullan was appointed Integrity Commissioner for the City of Toronto.