He was elected as a
Toronto City Council alderman in the
1966 municipal election. As an engineer, he focused on the need for a clearer understanding of the environment and, as a reformer, encouraged a new approach to local politics. He ran for
Toronto mayor in
1972, losing to
David Crombie, and again in
1978 to
John Sewell. His 1978 defeat to the left-wing Sewell was seen as a result of
vote splitting between O'Donohue and
David Smith, a
Liberal. O'Donohue and colleague
Art Eggleton had agreed that only one of them should run against Sewell for mayor in 1980. They were to conduct a public opinion poll to determine which of them had the better chance of toppling the incumbent Mayor. However, according to O'Donohue's memoirs, Eggleton broke the pact and unilaterally declared himself a mayoralty candidate forcing O'Donohue to stay out of the race in order not to split the vote. O'Donohue was returned to City Council following a by-election after the death of City Councillor
George Ben in 1980. He spent the next 14 years working to address the many energy and environmental problems facing urban areas. In April 1989, O'Donohue moved a by-law at City Council to ban the manufacture, sale, distribution and use of ozone depleting substances. It was the first such legislation anywhere and became a model for other cities. As a result, he was invited to make a presentation on the Toronto by-law at the Beckman Institute of the
National Academies of Science and Engineering, in
Irvine, California. While there, he consulted world-renowned chemist and later Nobel laureate
Frank Sherwood Rowland on how to help prevent the dumping of ozone depleting substances into the atmosphere. In 1992, he presented a motion to City Council to adopt a by-law prohibiting anyone from lying, sleeping or blocking city sidewalks. He argued that the city paid millions of dollars to make sufficient beds available for the homeless and there was no need for anyone to lie or sleep on the sidewalks. The motion lost and sleeping on the sidewalks has remained part of the landscape in downtown Toronto streets. In the
1994 Toronto municipal election, he was defeated in Ward 3 by 28-year-old
Mario Silva. The result was very close and subject to several recounts before the Ontario Court of Appeal ruled that Silva had won by 15 votes. ==After politics==