In 1982, Mayor Marion Dewar participated in the ribbon-cutting ceremony at Ottawa's first feminist bookstore, the Ottawa Women's Bookstore, signifying a period of growth in the women's lesbian and gay community and underscoring her commitment to addressing and promoting LGBT rights within the community.

Dewar was a peace activist and campaigner for nuclear disarmament, and, for example, picketed the U.S. Embassy in Ottawa after the invasion of Grenada.

She was elected to the House of Commons of Canada in a 1987 by-election in the riding of Hamilton Mountain, replacing NDP incumbent Ian Deans.

[9] Continuing to play a prominent role in the NDP, she was one of the leading backers of Audrey McLaughlin's leadership bid.

In addition to her political activities, Dewar did voluntary work for many community organisations, including the Ottawa Women's Credit Union.

[14] On Friday afternoon September 12, 2008, Dewar was rushed to a Toronto hospital after suffering a serious fall and subsequent brain hemorrhage.

Among the over 800 people attending the public service were notable politicians including Ontario Premier Dalton McGuinty.

[19] Marion Dewar Plaza, which provided a central site for festivals and special events, is located at 110 Laurier Avenue W., adjacent to the Ottawa City Hall complex.