The Jambos are closing in on their first title since 1960 and becoming the first side outside of the Old Firm to become champions for 41 years.
But Celtic remain in the hunt for their 14th title in 15 years after Iheanacho's spot-kick kept them only one point behind.
At a raucous Tynecastle, Hearts were hoping for a historic night in Edinburgh.
Derek McInnes' men were at one point on course for the title as Celtic trailed at Fir Park.
After a nervy start, two goals in five minutes kept the Hearts dream on track.
Centre-back Frankie Kent crashed home a header before Cammy Devlin's deflected shot doubled the lead.
Blair Spittal added a late third but all the drama was taking place in Motherwell.
Elliot Watt's effort gave the home side a deserved lead.
Celtic rallied either side of the break thanks to blistering strikes by Daizen Maeda and Benjamin Nygren.
Motherwell penned Martin O'Neill's men back and got their reward on 85 minutes when Liam Gordon swept home after a goalmouth scramble.
Celtic would have had to beat Hearts by three goals on the final day had they not found a winner.
Deep into stoppage time a VAR review spotted a raised arm by a Motherwell defender inside his own box.
Former Manchester City forward Iheanacho took responsibility to coolly slot home and set up a thrilling finale to a dramatic season.
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