Promising to be a "happy warrior" for Montrealers, Valérie Plante was sworn in on Thursday as the city's first female mayor in its 375-year history.

Plante received a standing ovation as her name was called and she was declared officially elected as the city's 45th mayor.

"A lot of people know that, but I'm a fighter," she told the appreciative crowd in Montreal's historic Bonsecours market.

"My nickname, for those who don't know it, I'm the happy warrior. So I might have a new role as mayor but I will always fight for you."

Plante, who caused a major surprise when she won more than 51 per cent of the vote to defeat incumbent Denis Coderre on Nov. 5, repeated the oath of office as she was sworn in along with her fellow councillors.

She was then invited to sign Montreal's official book, flanked by her husband Pierre-Antoine Harvey and her two sons.

In her speech, she described her administration's top priorities as mobility, road safety, affordable housing, public services and economic development.

She said Montreal represents much more than her office and that of city councillors.

"This city belongs to Montrealers," she said. "If you love it like I love it and if you want to see your children grow, as I'm seeing mine grow, I urge you to get involved.

"Montreal is made up of its neighbourhoods and the people who live in them. Those are the two main riches I will highlight during this mandate."

Plante, 43, entered municipal politics in 2013 when she won a council seat and she was elected leader of the left-leaning party Projet Montreal three years later.

Her signature campaign promise in this year's campaign was a new subway line that would link the city's densely populated northeast to downtown.

Plante, who was born in Rouyn-Noranda in northwestern Quebec, spent a year as a teenager in North Bay, Ont., to learn English.

She moved to Montreal at the age of 19 and attended university where she received degrees in anthropology and museology.

She then worked for a number of non-profit organizations.

Speaking to reporters following the swearing-in, she said one of her first moves as mayor would be to put more buses on the streets.

But when asked again about her first order of business, she stressed that her work in the city had already begun.

"The city was not waiting for the mayor to be sworn in," she said. "Business has already started."

Keep reading