McLellan earned bachelor's degrees in Arts and Law from Halifax's Dalhousie University. She then earned a Master of Laws from King's College London in the United Kingdom in 1975.
She became a professor of law, first at the University of New Brunswick and then, beginning in 1980, at the University of Alberta Faculty of Law where she served at various times as associate dean and dean. She has also served on the board of directors of the Canadian Civil Liberties Association.
Her first foray into politics was as the Liberal candidate for the riding of Edmonton Northwest in the 1993 general election, when she won her seat by 12 votes. She quickly became a rising star in the Liberal Party, being one of four Liberals elected in Alberta, and was named to cabinet as Minister of Natural Resources. McLellan has the prenominal "the Honourable" and the postnominal "PC" for life by virtue of being made a member of the Queen's Privy Council for Canada on November 4, 1993.
She was re-elected by narrow margins in the new riding of Edmonton West in the 1997 and 2000 elections, despite the Liberals' general unpopularity in Alberta. Her frequent narrow escapes gave her the nickname "Landslide Annie" in Canadian political circles.
McLellan served as Minister of Justice from 1997 to 2002, with responsibility for implementing new anti-terror and security laws following the September 11, 2001, attacks in the United States, and the implementation of the Canadian gun registry. She served as Minister of Health from 2002 to 2003.
Though she supported Paul Martin for the Liberal leadership, she was kept in Jean Chrétien's cabinet due to her ability and also because Chrétien wanted an Albertan in his cabinet for the sake of regional representation.
On being sworn-in as Prime Minister on December 12, 2003, Paul Martin named her his Deputy Prime Minister. McLellan was also named minister for the newly created Department of Public Safety and Emergency Preparedness. As Deputy Prime Minister, she was also chair of the Operations Committee of the Cabinet. McLellan's appointment was one of a number of women given senior positions in the Paul Martin government.
During the 2004 federal election, she was re-elected by 721 votes, or just over 1% of the vote, defeating Laurie Hawn of the Conservative Party of Canada in the riding of Edmonton Centre.
In the 2006 federal election, McLellan was defeated by Hawn 45.01% to 38.36%.
McLellan has so far been the last Deputy Prime Minister of Canada, as the succeeding Harper government has opted not to use this title.
McLellan is one of the few Canadian parliamentarians to have spent her entire career as a cabinet member.
On May 12, 2006, McLellan was appointed Distinguished Scholar in Residence to the University of Alberta at the Canadian university's Institute for United States Policy Studies. On June 27, 2006, she also became counsel to the Edmonton-based law firm Bennett Jones LLP. She also became a director on the boards of Nexen Inc., Agrium Inc. and Cameco Corporation.
On July 1, 2009, McLellan was appointed an Officer of the Order of Canada for her service as a politician and law professor, and for her contributions as a community volunteer. On May 9, 2013, she was appointed to the Alberta Order of Excellence for her achievements in politics, law and advanced education.
In 2016, McLellan was controversially appointed as the head of the Task Force on Marijuana Legalization and Regulation, created to provide recommendations on the design of a new system to legalize, strictly regulate and restrict recreational use of marijuana, despite her position within Bennett Jones. The process included an opportunity for the public to provide their own input. On 13 December 2016, the panel's report was released to the news media; its recommendations are not binding on the legislators.