The Buddies stamped their authority from the start, and it only took three minutes for their first sight of goal when Cammy Smith beat Andy Dowie to the ball on the left before cutting into the box and forcing Scott Gallacher into a smart stop. Saints, though, were not to be denied and they went ahead when Smith found Lewis Morgan on the edge of the area, and the on-loan Celtic man expertly curled the ball into the far corner.
Dumbarton had a chance to level with 27 minutes played but Kyle Hutton headed wide after escaping his marker from an Andy Stirling corner.
The Paisley side continued to apply pressure and almost went two in front when a Kyle Magennis corner eluded everyone and looked destined for the net before David Smith headed off the line.
But Saints didn’t have to wait long before doubling their lead, Danny Mullen crossing low for Smith who bundled the ball over the line to give the hosts a cushion with 32 minutes played.
The visitors looked for an immediate response after falling two behind when Stirling crossed for Kevin Nisbet but his shot was well turned behind by Craig Samson.
The Buddies could have extended their lead further as the first half drew to a close but Stephen McGinn fired wide before Ryan Flynn’s sliding effort flew over the bar.
Saints made the points safe on the restart when Mullen met a Cammy Smith pass with a near-post finish that gave Gallacher no chance.
Sons, perhaps suffering a hangover from their Irn-Bru Cup final exertions, had little to offer as the game wore on and it was little surprise when St Mirren extended their lead through substitute Gavin Reilly’s decisive 14-yard strike.
The champions-elect were still not finished and added lustre to a polished performance when Morgan took a pass from Cammy Smith and drilled an unstoppable shot high into the net from just outside the box.