Andrea Horwath celebrates after winning the leadership of the Ontario New Democrats on Saturday. (Cathie Coward/Hamilton Spectator/Canadian Press)

Hamilton MPP Andrea Horwath has been elected leader of the Ontario New Democrats.

Horwath, 46, became the first woman to lead the party after winning on the third ballot. She will succeed Howard Hampton, who is stepping down after 13 years as leader.

"First order of business is holding Dalton McGuinty to account in question period on Monday," Horwath said 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."

Horwath won with 60.4 per cent of the vote, beating Peter Tabuns, a former Greenpeace executive, on the final ballot. Horwath and Tabuns were the two remaining contenders in the race after Gilles Bisson was dropped after the second round of voting early Saturday.

Former East York mayor Michael Prue finished last on the first ballot with just 11 per cent of the vote. He threw his support behind Bisson.

The ballots were cast in Hamilton under a preferential voting system that asked voters to rank the candidates in order of preference.

The New Democratic Party has 10 seats in the 107-seat provincial legislature.

With files from the Canadian Press