William Donald Dennison (January 20, 1905 – May 2, 1981) was a Canadian social-democratic politician who served in both the Ontario Legislative Assembly and finally as the City of Toronto's mayor.

As a young man, he would trek west to Saskatchewan in the summers to earn money helping with the harvest and pitching grain.

On council he interrogated other politicians and officials on conflict of interest, expense accounts, and their relationships with companies doing business with the city.

[5] He ran to be Toronto's mayor in 1966, campaigning on providing "a strong voice for labour in city affairs" and opposing the pro-development policies of incumbent Philip Givens.

He decided not to run again for mayor, and due to a prostate operation, watched the 1972 municipal election from a bed at St. Michael's Hospital.

While vacationing in the United States, a medical emergency arose due to his Parkinson's disease, and it finally forced him to be evacuated back to Toronto in April 1981.

[10] Their only child, Lorna Dennison Milne, was a community activist who was appointed to the Senate of Canada, sitting in the Red Chamber as a Liberal from 1995 to 2009.