Hamilton MPP Andrea Horwath beat out three other caucus members to become the new leader of the Ontario NDP.

"First order of business is holding Dalton  McGuinty to account in question period on Monday," Horwath told reporters in Hamilton on Saturday following her acceptance speech.

"It's going to be an excellent opportunity for us to grow our party, so other than doing that work at Queen's Park I'll be working with our riding associations and I'll be working with Ontarians to try to make some positive change in this province."

McGuinty was quick to issue a statement, saying, "I know Ms. Horwath is hard-working, dedicated and committed to public service, and I look forward to working with her in her new role as leader."

Horwath will be the party's first female leader. She takes over from outgoing leader Howard Hampton, who served in the position for 13 years.

She defeated Toronto-Danforth MPP Peter Tabuns on the third ballot of the race. Northern MPP Gilles Bisson lost out on the second round of voting, and Toronto's Beaches-East York MPP Michel Prue lost on the first.

Prue threw his support to Bisson, who in turn threw his support to Horwath.

The battle that began eight months ago entered its end game at the Hamilton Convention Centre.

All NDP members were eligible to cast ballots, which they could do online or by phone (an estimated 1,000 were at the actual convention). First-round voting ended at 3:30 p.m. The voters were asked to rank the four candidates in order of preference.

There was no clear heir-apparent to Hampton, who took on the position in 1996. Hampton himself succeeded Bob Rae, the only man to serve as an NDP premier of Ontario to date. Rae is now a Toronto Liberal MP.

Party officials had suggested at least two rounds of voting would be required to select a winner. It ultimately took three. Unions held 25 per cent of the votes.

Tabuns and Horwath had been considered at the front of the pack going into the convention.

Tabuns, a former Toronto councillor who once headed Greenpeace Canada, had the support of some heavy hitters including former NDP leader Ed Broadbent, Parkdale-High Park MPP Cheri DiNovo, former Toronto MP Peggy Nash and social activist Michele Landsberg.

In congratulating Horwath, Tabuns said the NDP had "to continue speaking with a united voice to defend the principles and values that are at the core of our party."

Horwath, 46 and the mother of a teenaged boy, once served on Hamilton's city council and has worked as a community organizer. Her father had been a CAW member, and she enjoys the personal backing of Ontario Federation of Labour president Wayne Samuelson.

"She's got an incredible energy, she brings to this party a sense of direction," he said. "The delegates of this convention have seen the kind of leader, have seen the potential."

Prue had served as mayor of East York, before it was amalgamated into the current "megacity" of Toronto. He became an MPP in 2001, defeating Greenpeace founder Robert Hunter, who ran for the Liberals. He counted many elected Toronto politicians among his supporters.

But afterwards, he admitted his bid had been a long shot.

Bisson has been an MPP for 18 years, but worked as a labour union organizer prior to entering politics. He didn't serve in Bob Rae's cabinet, but acted as parliamentary assistant to the ministers of northern development and mines and francophone affairs. He has many supporters among elected NDP politicians from the province's north.

"What's important is that the party has to be unified and that's what we're getting here," Bisson said, hugging Horwath.

"Andrea (is) a person of integrity, I've always respected her, she's person who's got an ability to connect with voters."

Hampton, 56, will stay on as MPP for the northern riding of Kenora-Rainy River. During his tenure, the NDP failed to gain official party status in the 1999 and 2003 provincial elections.

He told reporters on Saturday that his party will play a role in a minority government following the 2011 provincial election.

"Bay Street has lots of people representing it," he said. "The only people (in the legislature) who want a world a little more fair and decent are New Democrats. Even people who don't vote for us say that's what the NDP is all about."

With files from The Canadian Press