Raised in the Beaches neighbourhood, in Toronto, Sewell attended Malvern Collegiate Institute and the University of Toronto from which he graduated with an English Literature degree in 1961. He earned a law degree from the University of Toronto Law School in 1964 and was called to the bar in 1966.
Sewell first became active in city politics in 1966 when he joined the residents of the Trefann Court Urban Renewal Area in the fight against the expropriation and levelling of the working class and poor neighbourhood. He was first elected to Toronto City Council in 1969 and became leader of the reform wing. He was elected Mayor of Toronto in 1978. In the 1978 election, the right-wing vote was split between two mayoral candidates: David Paul Smith and Tony O'Donohue. Sewell was able to win the election with less than 50% of the vote.
Sewell was portrayed as a radical in the media and was dubbed "Mayor Blue Jeans" by the Toronto Sun at a time when wearing denim was still considered an identifier of the counterculture. As an environmentalist famous for riding his bicycle to council, he opposed the development of banking and convention centres in the central business district that would become the hallmark of the mayors who followed. Sewell also established himself as a leading critic of the Toronto Police, demanding greater accountability to the public, and was a leading defender of gay rights, endorsing gay rights activist George Hislop's 1980 candidacy for city council, at a time when it was rare for public figures to express support for same-sex rights.
In the 1980 election, after two years of controversy, pro-development Conservatives and Liberals united behind the candidacy of Art Eggleton. Although Sewell maintained the support of many Red Tories, reform Liberals and New Democrats and won more votes and a larger share of the vote than in 1978, he lost the mayor's office to Eggleton.
Sewell subsequently returned to city council as an alderman in a by-election and won re-election in 1982. He retired from municipal politics in 1984 to accept a job as a columnist at the Globe and Mail. He subsequently moved to NOW Magazine, then wrote a regular column in Toronto's eye weekly from 1999 to 2005. He has written a number of books and articles on Toronto urban issues.
Sewell served as chair of the Toronto public housing authority from 1986 to 1988 and is an acknowledged urban affairs expert. He has served as chair of the Royal Commission on Planning and Development Reform in Ontario from 1991 to 1993. Sewell was an advisor to East London, South Africa's city council from 1994 to 1999 and as advisor on the re-establishment of local government in Malawi in 2000. Sewell also taught law, politics and social science at York University from 1989 to 1991.
In the late 1990s, Sewell founded the group Citizens for Local Democracy to fight the plans of the provincial Mike Harris government to abolish Metropolitan Toronto and amalgamate its constituent parts into a new City of Toronto "megacity".
In the 1999 Ontario provincial election, Sewell ran as an independent candidate in the riding of Toronto Centre--Rosedale, challenging Progressive Conservative cabinet minister Al Leach to protest the megacity. His entry into the race was controversial, with many activists accusing him of splitting the left-wing vote with the New Democratic Party (NDP). Sewell was also criticized for remaining in the race after Leach, whom he had personally targeted as the minister responsible for amalgamation, withdrew from the contest. The riding was ultimately won by Liberal George Smitherman; Sewell finished third, behind Tory Durhane Wong-Rieger.
On June 26 2006, Sewell announced that he would seek election in Ward 21 running against Joe Mihevc in Toronto's 2006 municipal election. Sewell said that he was motivated to run due to the construction of a streetcar right-of-way along St. Clair Avenue that is supported by Mihevc. He also stated that he is disappointed at the record of Mayor David Miller. "Living in a megacity demands more citizen participation and community consultation, not less," says Sewell. While his candidacy received much publicity in the local media, he was easily defeated by Mihevc.
Sewell, a former resident of Riverdale, currently resides in Ward 21.