The striker headed home the only goal of the rearranged Clydesdale Bank Premier League 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.
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 sending a cross to the back post for Porter to head into the corner.
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: Graeme Smith, Quinn, Craigan, Reynolds, Hammell, Klimpl, O’Brien (Murphy 87), Lasley, Hughes, Porter (Darren Smith 86), Clarkson.
HEARTS: Balogh, Neilson, Berra, Zaliukas, Lee Wallace (Cesnauskis 73), Jonsson, Stewart, Palazuelos, Driver, Nade (Elliot 63), Obua (Glen 68).
Referee: D Somers.