McGuinty entered Thursday's races needing wins in both Vaughan and Kitchener-Waterloo to return his government to the majority status lost in last October's provincial election.
As expected, Liberal candidate Steven Del Duca won in Vaughan, a longtime Liberal seat in the community just north of Toronto.
In the Kitchener-Waterloo riding, located about 90 minutes southwest of Toronto, NDP candidate Catherine Fife delivered a surprise win, beating PC Tracey Weiler, who placed second, and Liberal candidate Eric Davis who finished third.
The results mean McGuinty’s Liberals have 53 seats, one shy of the 54 needed for a majority and the same total they held before the byelections were called.
The Progressive Conservatives hold steady at 36 seats, while the New Democrats climb to 18.
Appearing at the victory podium with Del Duca in Vaughan, McGuinty conceded that Thursday’s results will force him to work with other parties at Queen’s Park.
"Winning a byelection in government is never an easy thing at the best of times and these are not the easiest of times," he said. "We will continue to work with the other two opposition parties, we will continue to find common ground."
Del Duca managed to hold off a challenge from PC candidate Tony Genco. Del Duca previously served as the executive assistant to former Vaughan MPP Greg Sorbara and he worked for McGuinty when he was leader of the Official Opposition.
Fife, the winner in Kitchener-Waterloo, is a former president of the Ontario School Boards Association. Teachers spent the last few weeks actively campaigning in the riding, angry over the McGuinty government's move to legislate a two-year wage freeze for teachers.
McGuinty engineered the byelection in K-W by appointing veteran PC Elizabeth Witmer to a $188,000-a-year post as head of the Workplace Safety and Insurance Board, convincing her to give up the seat she'd held for 22 years.
Progressive Conservative Leader Tim Hudak did not appear publicly Thursday night, but he issued a statement blaming unions for teaming up to help the New Democrats take Kitchener-Waterloo.
"Tonight's result has shown that public sector unions from across Ontario were provided with the perfect opportunity to concentrate their resources and lash out against the wage freeze we've been consistently pushing," said Hudak.
"They bought Kitchener-Waterloo, and now we can expect the rest of Ontario taxpayers to pay for it as the NDP cut more budget deals to keep the Liberals in power."