Selected as Britain's next prime minister in the very process she advised against, Theresa May will now have to negotiate a Brexit deal she didn't want.

May, 59, insisted to fellow Conservatives just last week their next leader "needs to have won a mandate to lead." She would have no part in a leadership coronation and preferred an "open, honest and robust debate" to choose David Cameron's successor.

But how things change rapidly in the roller-coaster ride of British politics.

May's only remaining opponent, Andrea Leadsom, quit the race on Monday, leaving the front door at 10 Downing Street wide open.

Gone was the promise of a "robust debate" that May endorsed just days ago. She will be Britain's 76th prime minister by Wednesday night. 

Between left and right

It's just the latest contradiction in May's 19 years as an MP, including the past six as home secretary, the top official in charge of public safety.

She's been described as tough on immigration, presenting plans to curb the flow of newcomers to Britain below 100,000 a year. May championed legislation to deport non-EU nationals (including, in theory, Canadians) living in Britain and earning less than £35,000 a year (that's nearly $60,000 CDN now, with the pound dropping post-Brexit vote).

'She's not coming in with a lot of friends.' -Catherine Meyer

Her views don't always skew to the right of the political spectrum, however. In 2012, she became the most senior British politician to take part in a video campaign in support of same-sex marriage.

"Because of her political positioning in the centre of the Tory party, she will be able to unite her party," said Sean O'Grady, associate editor of Britain's left-leaning online newspaper the Independent.

"I think she will find it much harder to unite Britain as a country," he said, after the divisive Brexit vote May campaigned against, albeit lukewarmly.

Stubborn: a key to success

Whatever her opinion on an issue, May is said to stick to her guns.

Tory MP Kenneth Clarke described May as a "bloody difficult woman" during the course of unguarded comments he made in a Sky News television studio earlier this month, apparently unaware the camera was rolling. 

The remark about May, deemed sexist and stereotypical, ignited a firestorm on social media. May ran with the characterization in subsequent comments to media and colleagues, but by then other women were already using the hashtag #BloodyDifficultWoman in a positive way.

"People see her as quite cold when they first meet her simply because she's a bit shy and a reserved person," said Catherine Meyer, a fellow Conservative who's known May for a decade.

"She's not coming in with a lot of friends," Meyer told CBC News. May "will appoint the right people [who] she thinks are the right people to be appointed, not because she owes something to somebody."

When gender plays a role

Meyer first met May at an event to promote the involvement of women in politics. She expects the appointment of a woman as prime minister — just Britain's second — will do wonders to attract women to politics.

"I was a bit disappointed with Margaret Thatcher," Meyer said, pointing to the former prime minister's few female cabinet appointments. May "will want to promote women for sure," she said.

May's gender came into sharp focus over the weekend, due to remarks by the other woman in the leadership race at the time. Leadsom told the Times newspaper she had a greater "stake" in the future of the country because she has children and May does not. 

Other politicians deemed the remarks mean-spirited and insulting. Leadsom said on Twitter that her comments had been misrepresented but later apologized to May, according to the Daily Telegraph on Monday. May recently revealed, an interview with the Daily Mail, the struggle she and her husband Philip faced trying unsuccessfully to conceive.

Longtime Conservative

May, the daughter of a vicar, has been involved in the Tory party for decades. Her official government bio says she began by "stuffing envelopes at her local Conservative association" before her election in 1986 as borough councillor in the London district of Merton. She was elected MP for Maidenhead in 1997. 

Oxford educated, she worked at the Bank of England and then held senior positions at the Association for Payment Clearing Services, set up by the U.K. banks in 1985 to manage credit, debit and other payment transactions.

It's not the typical background for someone who now says she wants to lead a country "that works not just for the privileged few."

Earlier Monday in Birmingham, England, before she knew with certainty that she would become prime minister, May even pitched herself as a champion for the working class.

"We need to reform the economy to allow more people to share in the country's prosperity," she said.

'Brexit means Brexit'

All her plans, though, will pale in comparison with the biggest project of her premiership: extricating the U.K. from the European Union.

"Brexit means Brexit," May said at least twice on Monday. And it's more than a catchy saying; it's a clear signal that May, who backed the Remain side, won't try to dilute the plan to leave the EU, respecting the will of a slim majority of voters.

She'll go through with it, just not right now. May has promised not to invoke Article 50 — the formal process by which a member state exits the EU — this year. Much preparation remains.

She's willing to use EU citizens as bargaining chips, not yet saying whether they'll be allowed to stay in Britain even if they were living here before the referendum. That will be up for discussion, May says.

Some parts of her Brexit negotiation strategy remain unclear, though.

In her first speech as Conservative leader on Monday afternoon, she spoke for one minute and 29 seconds and took no questions.

She'll have many to answer as the new occupant of 10 Downing Street.

With files from Reuters