William Donald Dennison (January 20, 1905 – May 2, 1981) was a Canadian social-democratic politician that served in both the Ontario Legislative Assembly and finally as the City of Toronto's mayor. He served two nonconsecutive terms as a Member of Provincial Parliament (MPP) in the 1940s and early 1950s. After his provincial-level career, he focused on Toronto's municipal politics, holding offices as an alderman, member of the Toronto Board of Control, and finally as the city's mayor. He was the mayor from 1967 to 1972, winning two consecutive three-year terms. Prior to entering politics, he was a school principal and teacher. As of 2020 he was the last mayor of Toronto to be a member of the Orange Order.