He served as the Official Opposition Critic for Foreign Affairs, until he left the post in October 2011 to run for the leadership of the NDP.

[6] Not long after he completed his Bachelor of Arts, Dewar embarked on a five-month trip to Nicaragua, where he volunteered as an aid worker.

During his time with the union, Dewar played a major role in reviving the OCETF's political action committee and in establishing the Elementary Teachers' Federation of Ontario's Humanity Fund, which provides financial support to various charities doing work in developing countries, such as the Stephen Lewis Foundation.

[12] Dewar was raised in a Catholic household, but he took issue with the church's positions on same-sex marriage, birth control, and women's rights.

[17] Several weeks after Dewar started his campaign, former NDP leader Ed Broadbent came out of retirement to also seek the nomination.

[19] Dewar won the proceeding nomination contest on June 22, 2005, on the first ballot, defeating NDP caucus Research and Communications Director Jamey Heath, who was the riding's candidate in the 1997 election, lawyer and film producer Tiffani Murray, and Ottawa Citizen automotive columnist Shannon Lee Mannion.

Beginning February 5, 2009, Dewar served as the Chair of the All-Party Parliamentary Group for the Prevention of Genocide and Other Crimes Against Humanity.

Dewar explained in media interviews that in his view it was important that the UN treaty cover all small arms because of the effects that they have had in conflicts in Africa.

[28] Dewar was one of thirteen Canadians banned from traveling to Russia under retaliatory sanctions imposed by Russian President Vladimir Putin in March 2014.

[31] Dewar was considered to be among the leading candidates in the seven-person race; however, his lack of fluency in French was seen as a major obstacle.