Liberal Bob Chiarelli won the provincial bylection in Ottawa West-Nepean riding on Thursday. (Frank Gunn/Canadian Press)

Former Ottawa mayor Bob Chiarelli is the new Liberal member for Ottawa West-Nepean, while former Brockville mayor Steve Clark is now the Conservative member for Leeds-Grenville following an Ontario byelection Thursday.

Chiarelli took 43.46 per cent of the vote, defeating Conservative Beth Graham, who had 38.99 per cent, Elections Ontario reported. The NDP's Pam FitzGerald finished third with 8.43 per cent and Mark MacKenzie of the Green Party had 8.31 per cent.

In his victory speech Thursday night, Chiarelli said the things he learned during the campaign will guide his work in the provincial legislature.

"I knocked on 9,000 doors, I probably spoke to 3,000 people and it was reinforced again and again that we need a health care system and we need seniors programs that are going to accommodate the tremendously changing demographics in this community," he said. "That's what I'm committed to, that's what I'm going to work towards."

The riding's former MPP, Liberal Jim Watson, quit the cabinet in February to run for Ottawa mayor.

Chiarelli served as MPP for Ottawa West from 1987 to 1997. Two years later, redistribution created the new riding of Ottawa West-Nepean from 75 per cent of Ottawa West, 40 per cent of Nepean and 20 per cent of Ottawa-Rideau.

In 1997, Chiarelli switched to municipal politics, serving as regional chair of Ottawa-Carleton. He was mayor of the newly amalgamated City of Ottawa from 2001 to 2006 but lost his run for a second term.

In Leeds-Grenville, the Conservatives' Clark defeated Liberal Stephen Mazurek in dominating fashion. Clark took 66.60 per cent of the vote while Mazurek drew just 20.05 per cent.

Neil Kudrinko of the Green Party won 7.70 per cent of the vote and New Democrat Steve Armstrong 5.18 per cent.

Clark will replaces veteran Conservative MPP Bob Runciman, who resigned after holding the seat for nearly 30 years. Runciman is now in the Senate.