A community activist, Julian was Executive Director of the Council of Canadians and later the Western Institute for the Deaf and Hard of Hearing. He was a leader in an unsuccessful effort to prevent the arrival of big box retail in New Westminster without public consultation. Ultimately, the city allowed a big box store which turned out to be a Wal-Mart store. Julian served as the Provincial Secretary for the Quebec section of the New Democratic Party in the 1990s. He also has been the National Policy Coordinator and Assistant and Acting Federal Secretary of the NDP. He has been a member of the NDP since he was 14 years old. In 2002, Julian ran for city council in New Westminster, British Columbia. He received 3,275 votes, losing a spot on the council by 74 votes. After losing his bid for city council, Julian ran for the New Democratic Party nomination in the riding of Burnaby—New Westminster, British Columbia. On March 7, 2004 Julian defeated Dave Mackinon to be the NDP's candidate in the 2004 federal election. Julian won the general election, defeating Mary Pynenburg of the Liberal Party of Canada by just 329 votes. In the federal election held on October 14, 2008, Julian won the riding of Burnaby—New Westminster by over 6,900 votes. Julian won the riding again in 2011 with 49.67% of the votes. He also co-founded the Save St. Mary's Hospital Community Coalition. He was a founding member of the B.C. Disability Employment Network and the Burnaby-New Westminster Council of Canadians. He has also volunteered for the local Emergency Social Services, for Royal City Soccer, East Burnaby Minor Baseball, the United Way, and the United Church of Canada. In the New Democratic Party Shadow Cabinet, Julian is the Energy and Natural Resources Critic. Julian previously served as the NDP critic for International Trade, Transportation, Persons with Disabilities, Treasury Board, Western Fisheries Critic, Industry, and the 2010 Vancouver-Whistler Olympics. Julian also served as the Deputy NDP Caucus Chair. During the 2011–12 NDP leadership race, Julian took over from candidate Peggy Nash to serve as the NDP's Finance Critic until the race was over, at which point Nash retook her spot and Julian was shifted to the lower-profile position of Energy Critic. Julian was vocal opponent to the Security and Prosperity Partnership that he believed threatened Canada's sovereignty through deep integration with the United States and Mexico. As NDP Transport Critic, Julian led the successful fight in the House of Commons to stop the SMS transport safety bill, which he believed to be an attempt to turn safety over to air transport companies themselves, something Julian termed "self-serve safety". Recently, Julian initiated an NDP task force that will meet and consult with diverse Canadian immigrant communities across the country, and to learn more about the challenges they face. The Georgia Straight newspaper has called Julian "one of the region's hardest working politicians". Julian ranked 3rd of 308 MPs in the 39th Parliament on bills, votes, and speeches. After the NDP's third-place performance in the 2015 federal election, Julian was re-elected and served as NDP House Leader. On December 21, 2016, Julian registered to run in the NDP leadership race to replace Tom Mulcair. He withdrew on July 5, 2017, after trailing the other candidates in fundraising. He subsequently endorsed Jagmeet Singh for leader. On January 31, 2018, Peter Julian was named finance critic in the NDP House of Commons shadow cabinet by party leader Jagmeet Singh. In addition, Julian was made House Leader of the NDP and energy critic on March 14, 2019.