Horwath got the support of 77 per cent of the NDP delegates after making a passionate speech to the convention in the morning, one percentage point higher than she got at the party's last leadership review in 2012.
"I first want to say how humbled I am by your vote of confidence and support tonight," she said to cheers from the packed convention hall. "I am proud as hell."
Horwath captured 811 votes while 244 delegates voted for a leadership election, admitting she hopes it puts an end to the debate about her supposed swing to the political right in the spring election campaign.
Delegates had earlier voted to delay the leadership balloting by eight hours instead of proceeding with it as scheduled right after Horwath's keynote address, and party insiders said that may actually have helped by giving the leader more time to win over supporters.
"I think delegates voted on the future, on what they want to see going forward, and not so much about the concerns that were raised by some delegates coming out of the election campaign," a beaming Horwath later told reporters.
"I feel very buoyed about the support I got from our delegates," she added. "I feel very excited about the future and I think that they sent a very clear message here that 'We are prepared to go forward with you as the leader.'"
Most of the 21 elected New Democrat caucus members had supported Horwath and there were no open challengers to her leadership, even though the unelected NDP Socialist Caucus had called for her resignation for running a right-wing campaign.
In her pre-vote speech, Horwath acknowledged there were concerns about the party's election campaign and spoke about core NDP values such as building a society that is socially and economically equal.
"Our cause is a province that cares about the air our children will breathe, the land they will walk and the water they will drink," she said. "A province with a living minimum wage, to that the very concept of working poor becomes a part of the past."
Horwath rejected suggestions from both the Liberals and Conservatives that New Democrats can't be trusted with the public's money.
"People who have treated themselves and their friends to a 20-year fiscal drunken orgy financed by debt and service cuts, to pay for billions of dollars in tax cuts for people who need them the least, have no business lecturing us about the budget," she said.
Horwath also went after the Liberal plan to sell off Hydro One's local electricity distribution arm, and warned it would force rates even higher while private investors in the utility get rich.
"Essential services like electricity (should) stay in the public sector and aren't eaten like oysters in a Bay Street bar for somebody's else's profit," she said.
Many New Democrats were still mad at Horwath for triggering the election by rejecting what they considered to be a very NDP-friendly and progressive budget from the Liberals, an idea the leader flatly dismissed.
"Let's call that budget what it is," she said. "An austerity budget and a four-year blueprint for cutbacks to basic health care all across Ontario."
Follow @CPnewsboy on Twitter
Also on HuffPost