She won by a significant margin in her Hamilton Centre riding, which has been solidly New Democrat territory for the past 50 years.
Horwath first won her seat in a May 2004 byelection, taking the riding from the governing Liberals at the time. She became NDP leader in 2009.
Her party is still trailing in third place as results come in from across the province.
The NDP held 21 seats in the legislature at dissolution.
Horwath triggered the snap election when she refused to support the minority Liberal government's budget at the beginning of May.
Through the campaign, Horwath repeatedly called the Liberals "corrupt" and said they couldn't be trusted to deliver on their promises.