Duncan made the news official on Twitter and Facebook on Wednesday night.
"It’s been the honour of a lifetime to have served my constituents in Windsor-Tecumseh and the people of Ontario," Duncan tweeted at 12:08 a.m. Thursday morning.
The tweet linked to his lengthy farewell posted on Facebook. In that message, Duncan said his retirement is effective Feb. 14.
"It has been an enormous privilege to serve in public office for nearly 25 years," Duncan wrote in the message posted on his Facebook page at midnight Thursday. "Most of all, I have had the opportunity and pleasure to know, and work with, literally hundreds of amazing people from every walk of life, and yes, of every political stripe. I am proud to have served my constituents in Windsor and Tecumseh, and the people of Ontario as a whole."
But the 54-year-old may not be finished with politics just yet. In published reports Thursday, he hinted at running federally.
"There are other personal developments, including my next career, which I will have more to say about in the days and weeks ahead," was all Duncan said on his Facebook page.
Duncan made it clear after Dalton McGuinty announced he was stepping down as Liberal leader that he would follow McGuinty's cue and leave provincial politics.
Duncan has been an MPP since 1995 and has won five straight elections.
The decision means incoming Premier Kathleen Wynne will be faced with at least one by-election — though some analysts believe more Liberals will resign in the next few weeks.
Duncan said he would not comment on what he will do in the future until after he has officially resigned.