Tom Walsh returned to Paisley to knock St Mirren off the top of the Championship with a fabulous winner ten minutes from time.
And it was everything Dumbarton deserved against a surprisingly poor St Mirren side, who were always struggling after Stelios Demetriou was sent off just before half-time.
The Cypriot midfielder hit Sam Wardrop on the face with a flying elbow and Andrew Dallas had no hesitation in showing a red card.
Saints manager Jack Ross was not happy with Demetriou but he was angrier about the overall performance as Dundee United reclaimed top spot.
“When you’re not playing well and you lose a man it makes it even more difficult,” Ross said.
“It was an ill-judged decision by him to make that challenge and his punishment will come in how he has let his team-mates down and the games he will miss.
“He apologised but the game is done. He has to learn from it.
“I have no complaints over the result. We were a long way away from the levels we are normally at. We’ve grown as a side over the last year and we’re going to have bumps. It’s never going to be perfect.
“But I was disappointed in the players. They knew the significance of continuing to win games and there is no excuse for it.”
Walsh spent the first half of last season with St Mirren, scoring four goals in 15 appearances, and he was one of five former Saints in the Dumbarton line-up who were clearly inspired to do well against their former employer.
He was excellent throughout, although he missed a good chance in 11 minutes. Ally Roy should have scored from his pass in 73 minutes but shot wide.
However, Walsh sealed it in 80 minutes when he brilliantly raced away from defender Adam Eckersley and clinically drove a left foot shot into the right corner.
Stevie Aitken, the Dumbarton manager, said: “Tom has been terrific for a number of weeks now and, along with a lot of our players, shouldn’t be playing at this level of football.
“I think he has all the ability in the world and he took his goal really well.
“I think you always try to do well against your former club and I’m sure Tom will be delighted with the way he played. A lot might be made about the sending off but when they had 11 we were really strong in the game. I think we deserved it.”