Riordan received his PhD in Anthropology from Washington State University in 1984. He has worked extensively in the West, Midwest, and South pursuing an interest in the historical archaeology of the colonial period. He came to Historic St. Mary's City in 1985 and, as chief archaeologist, directs excavations for the museum. In 1992 he was co-director of Project Lead Coffins, recovering the burial caskets of the Calvert family at St. Mary's City. Currently he is directing the archaeological investigation of William Nuthead's print shop--the first print shop in Maryland.