He represented Barrie in the House of Commons until 2015, when he was elected as leader of the Ontario Progressive Conservative (PC) Party and resigned his seat in Parliament.

[citation needed] Brown served two terms as president of the Progressive Conservative Youth Federation (PCYF),[8] a position he held from 1998 to 2002.

[9] In the lead up to the 2004 federal election, Brown put his name forward as a candidate for the Conservative Party nomination race for the riding of Barrie.

[17] At the time of his jump to provincial politics, he chaired the Conservative Party of Canada's Greater Toronto Area caucus and the Canada-India Parliamentary Association.

[20] The game has featured NHL players like Connor McDavid, Aaron Ekblad, Steven Stamkos, Rick Nash, Brian Little, Mark Scheifele, and Andrew Mangiapane; former NHLers such as Wendell Clark, Darcy Tucker, Mike Gartner, Dale Hawerchuk, Bernie Nicholls, Gary Roberts, and Shane Corson; and has featured personalities like Prime Minister Stephen Harper, Hockey Night in Canada's Don Cherry and Ron Maclean, Ontario journalist Steve Paikin, Alan Thicke, Walter Gretzky and Jennifer Robinson, among others.

[21][22][23][24][20] In 2018, Brown passed on organizational leadership to Barrie-Springwater-Oro-Medonte MP Alex Nuttall for its eleventh annual charity hockey game.

[36][37] During Brown's leadership bid both special interest groups actively supported him by selling Ontario PC Party memberships amongst their members.

[citation needed] On May 9, 2015, Brown was elected leader, defeating his only remaining opponent, Christine Elliott, winning with 61.8 per cent of the membership vote.

[44][45] Brown resigned his seat in the House of Commons on May 13, 2015, after winning the leadership and led the Progressive Conservative party from outside the legislature during most of the summer.

[53][54][55][56][57] A day later, fellow MPP, Lisa MacLeod revealed that she heard rumours about similar allegations from former National Hockey League player Eric Lindros.

Dimitri Soudas, former director of communications to Prime Minister Stephen Harper and volunteer for the campaign, suggested the allegations were baseless, saying, “All media organizations were turning over every single stone that they could find, and they couldn’t find anything.”[58] A Globe and Mail article revealed that Brown's staff had expressed similar concern with Brown's past relationships after he became leader.

In addition, Fedeli announced that he would not sign Brown's nomination papers if he attempted to run in a Barrie riding at the June 7 provincial election.

The accuser alleged that she was an 18-year-old high school student when the oral sex incident occurred; but on February 13, three weeks after the first public report, she amended her claim to say she was a year older than she had previously maintained, and was therefore over the legal drinking age.

[74] Brown was ejected from the PC caucus on February 16, 2018, following reports that he was claiming he hadn't actually resigned as party leader and a series of Facebook posts in which he attacked the credibility of his accusers.

[77] On February 20, PC MPP Randy Hillier filed complaints to the Ontario integrity commissioner alleging that Brown had breached the province's ethics rules around down payments over his 2016 purchase of a $2.3 million waterfront house on Lake Simcoe, a loan from a friend and undeclared travels.

[83] The Progressive Conservative Party's nomination committee unanimously ruled on March 15, 2018, that Patrick Brown would not be eligible to run as a PC candidate in Barrie—Springwater—Oro-Medonte for the June 7, 2018 election.

[85] It was later reported by the National Post that a Snover Dhillon, a convicted fraudster banned by the federal Conservatives, played a role in all those nomination irregularities.

[90] His "Public Safety Initiative" would include more front line police officers, strengthening the mental health program, and affordable recreation for youth.

[94] In June 2019, Brown and Brampton City Council launched a public pressure campaign demanding greater healthcare funding from the provincial government.

[99] The game saw participation from NHLers, Jason Spezza, Sean Monahan, Malcolm Subban, Connor Brown, Casey Cizikas and personalities such as Nav Bhatia.

[105] After the 2024 National Security and Intelligence Committee of Parliamentarians report indicating that there was foreign interference in the Conservative leadership race, the Toronto Star revealed that Brown was probably negatively impacted by it.

[34] He spoke in the Legislature in support of a motion condemning Islamophobia,[117][118] After winning the leadership race, he focused his plan on four main issues which he suggested would lead to a more prosperous province: less red tape, improved transportation corridors, affordable energy, and addressing Ontario's growing skills gap.

[122] The bill was supported by Brown's fellow PCs but opposed by the governing Liberals and third party New Democrats, and was voted down 51-24 on division at second reading.

His decision to do so marked the first time a PC leader has ever officially marched in the major Toronto Pride parade event and signified an incremental evolution of the provincial party's history with the LGBTQ+ community.

[132] In an interview with the Canadian Press, Brown stated, “Any policy that attempts to limit a woman’s right to choose or the ability of same-sex couples to marry are off limits, period.”[133] In addressing the prospect of losing members of the party because of this more affirmative tone for the Ontario PCs, Brown said, “Frankly, I think I opened the party up to tens of thousands more who simply want a reasonable, thoughtful, modern, inclusive PC party,” so I'm not worried about a few leaving.”[133] As Mayor of Brampton in June of 2019, Brown wrote an op-ed in the Toronto Star discussing the historical and political importance of the Toronto’s Pride Parade and called on provincial and federal conservative leaders including Doug Ford and Andrew Scheer to officially march in the Toronto and Ottawa parades.

[134] In June 2019, Brown and Brampton City Council passed a unanimous motion to support a legal challenge to Quebec's Bill 21, a secularism based law that would make it illegal for government employees in positions of authority (public school teachers, police officers, judges, etc.)

[135] In explaining the justification for passing this motion as a municipality outside of Quebec, Brown stated, “Brampton is Canada’s most diverse big city.

[140] On January 22, 2024, Brown initially planned to issue a proclamation for opening Ayodhya Ram Mandir, a temple built on the site of the destroyed Mosque Babri Masji.

[147] In a video privately shared to members of the Tamil Canadian community, he stated that if he became Prime Minister, he would lift the Canadian terrorist designation of the LTTE and apologize for its listing, arguing that the LTTE's actions were "acts of self-defence against a Sri Lankan government that was acting in a manner that was a modern atrocity" and that "rather than it being a symbol of terrorism, the tiger is a symbol of, in my opinion, self-defence against the government committing war crimes."

"[148] At his first Ontario PC Convention as the new leader, Brown confirmed his belief in man-made climate change and announced his support for a revenue-neutral price on carbon.