OTTAWA — Doug Ford, a brash populist and a brother of former Mayor Rob Ford of Toronto, will become the next premier of Ontario, Canada’s most populous province, after leading his party to a decisive victory in elections on Thursday.

Just 21 minutes after most polls closed, both the Canadian Broadcasting Corporation and CTV News declared that Mr. Ford’s Progressive Conservative Party would gain a majority of seats in Ontario’s next provincial legislature.

Mr. Ford, who declared victory in a speech to supporters Thursday night, has led the party and been involved in provincial politics for only a few weeks. A populist who campaigned on pledges to reduce the size of government and cut taxes — and bring back $1-a-bottle beer — he represents a significant change for Ontario, which has been under the control of the Liberal Party for almost 15 years.

“My friends, help is here,” Mr. Ford told the crowd in Toronto. “Tonight we have sent a clear message to the world: Ontario is open for business.”