"Let's just say I've never been shy about going into the corners and I usually come out with the puck,'' Mulcair told The Canadian Press.
His relish of a scrap became the stuff of lore and fodder for his foes. It was once said of Mulcair that he could be the lone soul in an empty speakeasy and still start a bar fight.
His sharp-elbowed demeanour has made him few friends among long-time New Democrat MPs since he arrived in Ottawa in 2007, or in Quebec City during his noisy exit from Premier Jean Charest's Liberal government. But it did endear him to Conservatives, who tried unsuccessfully to woo him to join Stephen Harper's team.
His self-discipline in keeping that reflexive combative streak in check during an arduous six-month leadership campaign allowed him to claim the helm of the NDP Saturday after a day-long, four-ballot voting marathon.
Mulcair was born more than 57 years ago to Harry Donnelly Mulcair, an Irish-Canadian father, and Jeanne Hurtubise, a French-Canadian mother. He came to this world in an Ottawa hospital, but he grew up in Quebec and came of age in Laval, the cookie-cutter bicultural suburb north of Montreal.
As a youth, he seemed to effortlessly float from francophone to anglophone identity, depending on which linguistic crew he was in at any given moment.
His family was large and close. He was the second of 10 children; when asked about a sister helping him out on his leadership campaign, he quickly asked: "Which one?"
"I've got six sisters and I've probably slept on all their couches."
Mulcair had a genetic disposition towards a political career. His great-great grandfather was Honore Mercier, who was the premier of Quebec from 1887 to 1891.
He leapt into politics early. He was elected president of the McGill Law Students Association in 1976 — the same year he married Catherine Pinhas, a psychologist from France. He was able to get French citizenship through his wife.
The Conservatives, who made devastating use of ex-Liberal leader Michael Ignatieff's absence from Canada for over three decades, pounced on the perception that the next leader of the official Opposition might have divided loyalties.
"Obviously it's for Mr. Mulcair to use his political judgment in this case,'' Prime Minister Stephen Harper said. "In my case, as I say, I'm very clear. I'm a Canadian and only a Canadian.''
Mulcair, who bears bright scars from the bruising politics of Quebec's savage federalist/separatist divide carved out of two referendum campaigns, was incredulous.
"I have no lessons to receive about being Canadian from Stephen Harper.''
After getting his law degree, Mulcair moved to Quebec and worked for the government defending the French language — further evidence of his divided loyalties, Conservatives will surely insist. Except he followed that work up with a stint as legal director for Quebec's English-rights lobby group, Alliance Quebec, beginning in 1983.
He entered Quebec provincial politics in 1993 as a Liberal, and when Jean Charest formed a government a decade later, he was made minister of environment. His years as a Quebec Liberal were reason alone for many New Democrats to be wary of his leadership bid.
When rival contender Brian Topp nixed the idea of merging with the Liberals by arguing that New Democrats don't need to become Liberals in order to win government, Mulcair said it was "a coded message to the (NDP) base,'' intended to remind party members that he was a provincial Liberal until joining the federal NDP in 2007.
Mulcair insisted he had no choice but to be Liberal because that was the only federalist party in Quebec provincial politics, a big tent which encompasses people who vote Conservative, Liberal, NDP and Greens federally.
His departure from the Charest cabinet in 2006 was cantankerous. Charest said he quit rather than accept a demotion. Mulcair said he quit on a question of environmental principle: his refusal to greenlight development in a provincial park. Quebec Liberals whispered he wasn't a team player — whispers that supporters of his NDP leadership rivals were only to happy to pour into the ears of Ottawa reporters behind the veil of anonymity.
Mulcair's absence from politics was brief and his return was spectacular. Over dinner with Layton and their wives, the late NDP leader pitched the former Quebec Liberal on joining the NDP. After lengthy discussions with the Conservatives, Mulcair opted for the NDP and was not handed a soft landing.
Layton wanted him to run in Outremont, which had been in the hands of the federal Liberals — except for a very brief interregnum — for decades. He won in 2007 and, as the lone NDP Quebec MP, used Outremont as the forward operating base of his party's improbable breakthrough in the province four years later.
When a stunning 58 New Democrats were elected in Quebec in 2011 with Mulcair's help, he had a ready-made network of support and a solid chunk of caucus backing to launch his leadership bid after Layton succumbed to cancer.
The brawler was on his way to becoming the boss.