Former United player Steven Thompson gave St Mirren the lead in the 12th minute with a simple finish after good work from Nigel Hasselbaink.
United missed a golden chance to equalise minutes later when Keith Watson took a fresh air swipe at the ball with an open goal to aim at.
St Mirren were reduced to ten men in first-half stoppage time when Hasselbaink was sent off for serious foul play when he appeared to catch Paul Dixon with his arm.
United applied the pressure after the break and got the equaliser when Scott Allan crossed for Jon Daly to knock home from close range.
Dusan Pernis was fortunate not to concede a second goal for United when his clearance clattered off an opponent and flew into the side netting but the home side held on to claim the point that sees them top the SPL.
A last minute goal saw Dunfermline claim a point in a 3-3 draw with Inverness Caledonian Thistle at East End Park.
Dunfermline took the lead midway through the first-half when a long ball caused havoc in the Inverness defence and Andy Barrowman headed the ball on to Andy Kirk, who volleyed past Ryan Esson.
Inverness equalised shortly after the break with their first goal of the season thanks to a deflected Greg Tansey effort but Kirk headed home his second of the game minutes to put the Pars in front again.
Two goals in three minutes helped Inverness get their noses in front for the first time.
Johnny Hayes got the first when he slid the ball home from close range after Dunfermline keeper Paul Gallacher spilled Owain Tudur Jones' shot before Tansey bravely headed home his second as Inverness looked to have picked up their first win of the season.
However there was more drama to come when Martin Hardie fired a superb free-kick into the top corner to grab a last gasp point for Dunfermline.
Celtic are away to Aberdeen on Sunday while Old Firm rivals Rangers were excused SPL duty as they lost 3-1 to English Premier League side Chelsea at Ibrox.