The striker headed home the only goal of the rearranged clash at the end of a flowing move in the 11th minute.
Hearts then failed to trouble Motherwell goalkeeper Graeme Smith as the home side effectively closed down their visitors.
Hearts manager Csaba Laszlo had angered his counterpart Mark McGhee on the eve of the game by revealing Porter was on his list of transfer targets.
If the declaration was designed to unsettle the Englishman, it backfired spectacularly as Porter put a major dent in Hearts' European hopes.
Motherwell have now taken seven points from three games after a poor run which saw them fall to joint-bottom of the league.
The victory puts them eight points off fourth-placed Hearts and a potential UEFA Cup spot and they look to have rediscovered the confidence which helped them to a European place last season.
The unchanged home side had enjoyed the territorial advantage in a scrappy opening but they produced a slick move to take the lead.
The rejuvenated Jim O'Brien's backheel found Stephen Hughes and he turned a defender inside the box following a one-two with Paul Quinn, before standing up a cross to the back post for Porter to head into the corner.
Hughes almost turned another Hearts defender inside the box moments later before David Clarkson took down a high ball and volleyed wide as Motherwell looked to press home their advantage.
The visitors had failed to score in their three previous games and were missing as an attacking force in the opening quarter.
They had been forced into three changes after Christos Karipidis, Jason Thomson and Laryea Kingston picked up injuries in the draw with Hibernian, but they were able to bring back Marius Zaliukas, Lee Wallace and David Obua following suspension and injury.
They eventually began to exert some pressure but Motherwell worked hard to prevent them breaking through and closed them down in determined fashion.
Hearts were handed a chance to put the ball into the box when they were awarded a free-kick 30 yards out but Christian Nade headed two yards over from Andrew Driver's delivery, and 'Well comfortably took their lead into the break.
Smith made his first save 30 seconds after the restart when he gathered Eggert Jonsson's 30-yard drive, but other than collecting some cross balls, he had no others to make.
Hearts sporadically threatened with corners but the only time they produced an effort at goal was when Obua headed harmlessly over from Driver's delivery.
Motherwell looked more likely to add to the scoring, although they were guilty of over-elaborating at the end of some promising passing moves.
The ineffective front pairing of Nade and Obua were replaced by Gary Glen and Calum Elliot, who returned to Hearts this week after scoring 11 games in 14 league games on loan for Livingston.
Stephen Craigan, making his 250th appearance for Motherwell, was booked for bringing down Elliot on the touchline as Hearts pushed more men forward.
But Motherwell continued to defend well until the end and held on, although their deserved win was tainted slightly by the concerning sight of Porter limping off with five minutes to go.