The Paisley men twice led and looked capable of securing a vital win but the resilient Sons refused to accept second best.
St Mirren thought they had broken the deadlock when Cammy Smith’s deflected shot headed for the far corner of the net only for Daniel Harvie to produce a last-ditch goal-line clearance.
Dumbarton began to knock the ball around well and they were almost rewarded when David Smith’s cross picked out Thomson on the six-yard line. However, Gary Irvine came to Saints’ rescue with a superb block tackle.
The Buddies promptly broke upfield to take the lead, Smith’s slick cross finding David Clarkson, who produced a deadly first-time finish.
Christian Nade equalised after the break when he met Andy Stirling’s corner with a bullet header which gave Billy O’Brien no chance. He almost had a second but fired wide after latching on to a clearance.
Instead Saints restored their lead when debutant Harry Davis rose above the home defence to head Stephen McGinn’s corner high into the net.
A priceless win looked on the cards, but Dumbarton had other ideas and they levelled again when Thomson reacted first to Christian Nade’s knock-down to poke the ball into the net.