Though polls early in the 2015 federal election campaign indicated the possibility of an NDP minority government, the party lost just over half of its seats and resumed third-place status.

[6] His father, Harry Donnelly Mulcair, worked in insurance and was the descendant of Irish immigrants who arrived in the Quebec City area during Great Famine (Ireland) of the 1840s.

His mother, Jeanne Hurtubise, a school teacher, was French Canadian and the great-granddaughter of Quebec Premiers Pierre-Joseph-Olivier Chauveau and Honoré Mercier.

[7][8][9] The Mulcairs soon moved to the middle-class district of Chomedey in Laval, a suburb of Montreal, where Thomas would grow up as the second-eldest in a close-knit family of ten children.

The oldest, Matt, is a sergeant in the Sûreté du Québec (Quebec provincial police) and married to Jasmyne Côté, an elementary school teacher; they have two children, Juliette and Raphaël.

Mulcair also taught law courses to non-law students at Concordia University (1984), at the Saint Lawrence Campus of Champlain Regional College in Sainte-Foy, and at the Université du Québec à Trois-Rivières.

[10][15][29] Mulcair was president of the Office des professions du Québec (1987 to 1993), where he introduced reforms to make disciplinary hearings more transparent and successfully led a major effort to have cases of alleged sexual abuse of patients decisively dealt with.

[30][31] Mulcair was also a board member of the group Conseil de la langue française, and at the time of his appointment to the Office des Professions he had been serving as president of the English speaking Catholic Council.

[33] According to Le Devoir journalist Michel David, Mulcair is the person who coined the expression Pinocchio syndrome, which was the title of a book by André Pratte published in 1997 about lies in politics.

At the time of his appointment to Cabinet he had been serving on several volunteer boards including The Montreal Oral School for the Deaf, Operation Enfant Soleil and the Saint-Patrick's Society.

In addition, Coulon's writings had been condemned by B'nai Brith Canada; the local Jewish community in Outremont made up 10 per cent of the riding demographics.

Mulcair stated that Layton's death had hit him exceptionally hard, and that while he was considering a federal NDP leadership bid, he would need several weeks to make up his mind on that decision.

Thunder Bay—Superior North MP Bruce Hyer opted to sit as an independent after being disciplined for voting in favour of the dissolution of the Canadian Firearms Registry, a position counter to one strongly championed by Mulcair.

[71] Jonquière—Alma MP Claude Patry later defected to the Bloc Québécois after disagreeing with the NDP's position to amend the Clarity Act, another policy which was strongly promoted by Mulcair.

[78] Nevertheless, following the election of Justin Trudeau as Liberal leader in April 2013 the political fortunes of the NDP appeared to be on the decline, with the party falling back to its traditional third place in public opinion polls.

[79] The party would go on to lose a June 2014 by-election to the Liberals in the previously safe riding of Trinity—Spadina, which was made vacant following incumbent Olivia Chow's decision to run unsuccessfully in the 2014 Toronto mayoral election.

[88] Despite early campaign polls showing an NDP lead, the party lost 51 seats on election night and fell back to its former third place in Parliament.

[91] Following the election, Mulcair was criticized by some NDP members for having run on a moderate platform and promised to balance the federal budget whilst Trudeau's Liberals promised to run budget deficits in order to pay for social spending and economic stimulus programs; a position that was viewed as allowing the Liberals to outflank the NDP on the left of the political scale.

[98] In the days prior to the leadership vote, Mulcair confirmed his intention not to stand for parliament in the next federal election, expected in 2019, and suggested that he may resign his seat in the House of Commons as early as Christmas 2017 to accept one of the university appointments that has been offered to him.

Mulcair is pro-choice, and has stated at a conference in Quebec that people with an anti-abortion stance are not welcome to run for NDP, saying, "It's not debatable, it's not negotiable, it is a woman's right to determine her own health questions and her reproductive choices.

[103] He said an NDP government would "create an innovation tax credit to encourage manufacturers to invest in machinery, equipment and property used in research and development".

[105] Mulcair has promised to use additional tax revenue to pay for infrastructure, public transit, a new child care program, and a balanced budget.

[28] In response to the Idle No More movement, Mulcair said that the NDP would put a filter on decisions made to ensure that they respect court rulings and international obligations to Indigenous peoples in Canada.

[106] He also pledged to call a national public inquiry into missing and murdered aboriginal women within 100 days of taking office, if his party is elected.

[109] He also “criticized previous governments for allowing rail companies to police their own safety and called on Ottawa to take a more active role in doing that job” after the Lac Mégantic tragedy in Quebec.

[116] During a policy speech in May 2015, Mulcair announced the NDP would "increase overall funding for development assistance and ensure that poverty alleviation remains at the centre of Canadian aid efforts".

[125] On January 11, 2018, Mulcair assumed the volunteer position of chair of the board of Jour de la terre Québec, a non-profit organization dedicated to environmental issues.

[127] On July 17, 2018 Mulcair also announced that he had accepted a position as political analyst on Montreal talk radio station CJAD effective August 28, 2018.

[101] He will also appear on CTV News Channel (owned by CJAD's parent company Bell Media) starting in fall 2018,[128] and on the French-language network TVA in a similar capacity.

[129] As a consultant hired by homeopathy giant Boiron, Mulcair launched in November 2019 the company’s public relations campaign to get the Quebec government to create a professional order for homeopaths.