Brown, 36, a three-term MP from Barrie, Ont., defeated Christine Elliott, the widow of former finance minister Jim Flaherty and the deputy party leader, who had been endorsed by most of the PC caucus.
Brown, who doesn't have a seat in the Ontario legislature, dismissed an earlier warning from Elliott that social conservatives would take over the party if he won, and he promised not to reopen the abortion debate.
Convincing the majority of Progressive Conservatives that he isn't too far to the political right will help sway other voters too, added Brown.
"The first test of this leadership was to convince a majority of the members that I would be a pragmatic conservative that would lead our party to victory, and I think the size of my mandate here today proves that we've achieved that," he said.
Brown defeated Elliott by more than 1,500 electoral points in the final count that assigned 100 points to every Ontario riding.
Elliott, 60, asked the party members to make Brown the unanimous choice, and the PC caucus stood behind their new leader on the stage.
Brown started building a new Progressive Conservative party by selling 41,000 memberships — well over half the 76,500 total — and targeting multicultural communities in the suburbs around Toronto as well as reaching out to unions and other non-traditional Tory supporters.
"We are younger. We are more diverse," he said to cheers from the 800 party faithful that showed up at a Toronto convention centre to hear the results of voting that took place earlier in the week.
"We represent more points of view, more communities, more languages, more neighbourhoods and more cultures than ever before."
Brown said he wasn't involved in any of the PC's past "policy disasters" such as the 2014 campaign pledge to cut 100,000 public sector jobs, and can bring a fresh start to the party.
"Labour leaders, public and private sector, were actively engaged in supporting my candidacy," he said. "We had firefighters and nurses selling memberships."
Only five of the 28 PC caucus members supported Brown, but the avid hockey player who helps organize a charity game every year featuring NHL stars did have his campaign endorsed by Wayne Gretzky.
Toby Barrett said he was the last Tory MPP to endorse a candidate because he wanted to hear who his constituents wanted as leader, and he decided Brown has the personal touch needed to change the party.
"He's really one of us and reminds me in a way of (former premier) Mike Harris," said Barrett. "We saw Mike Harris and said 'Geeze, he's no different that us,' and I see that with Patrick."
The new PC leader said he wouldn't be holding "any grudges" against Elliott and the caucus members who backed her campaign.
Brown also said he would speak with Prime Minister Stephen Harper as soon as possible about resigning his seat in Parliament, but wouldn't say how quickly he would try to win a seat in the legislature. He did say it would be before the 2018 provincial election.
Brown also said his victory in the PC race "bodes well" for his federal Conservative colleagues in their upcoming election later this year.
Brown, who is single, was elected to city council in Barrie, Ont., while still going to university and won a seat in the House of Commons in 2006 as a Conservative MP, but was never appointed to cabinet.
He had virtually no provincial profile until he launched his leadership bid by attacking the Tory establishment he blamed for four consecutive election losses, saying the "grassroots wants its party back."
The governing Liberals wouldn't say if they're much happier at the prospect of facing Brown in the 2018 election than the more moderate Elliott, but suggested he is too extreme for Ontario.
"He is fundamentally a radical Tea Party individual who is far outside the mainstream," said Liberal cabinet minister Steven Del Duca.
The New Democrats said the fact Brown doesn't have a seat in the legislature makes them Ontario's real opposition party.
Brown will replace former PC leader Tim Hudak, who resigned shortly after the party lost its second election to the Liberals with him at the helm last June.
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