The Steelmen scored the only goal of the match after just three minutes at Love Street.
Chris Porter and David Clarkson played a neat one-two on the edge of the box before Clarkson sent a perfect ball into the six-yard area where former St Mirren man Steven McGarry was on hand to hammer the ball home.
Seven minutes later it was almost 2-0 with McGarry this time the provider. His right-wing cross was met by the head of Clarkson ten yards out but his effort hit the crossbar.
With five minutes to go until the break, Motherwell were handed a great chance to double their lead when they were awarded a penalty after McGarry's free-kick was handled by Billy Mehmet in the Saints wall.
However Jamie Paterson's spot-kick, although hit well, flew wide of the left-hand post.
St Mirren created a couple of early second-half chances with Mark Corcoran having a free-kick charged down and Hugh Murray seeing his header easily saved.
As the game moved into the final 15 minutes Clarkson had two chances to seal the game for the visitors. Firstly, he was blocked by Chris Smith and then sent a shot on the turn into the side-netting.
Motherwell held on for their first opening day win since 1998 and were rightly cheered from the field by their large travelling support.