In 1938, he was elected a school trustee and served three successive one-year terms. In 1941 and 1943 he won election to serve as an alderman on Toronto City Council for Ward 2 (Cabbagetown and Rosedale) After a ten-year interlude with his involvement in provincial politics, Dennison returned to Toronto City Council in 1953 serving again as an alderman for Ward 2. In 1958, he was elected to the Toronto Board of Control. On council he interrogated other politicians and officials on conflict of interest, expense accounts, and their relationships with companies doing business with the city. 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 Phil Givens. He was elected despite being opposed by all three daily newspapers. He was the first member of the CCF or NDP to serve as mayor of Toronto since James Simpson in 1935, and the last until Barbara Hall.