Calum Gallagher opened the scoring for the part-timers before Craig Barr’s second-half double rounded off a miserable day for an uninspiring United side that had levelled the scores twice through Anthony Ralston and Scott McDonald.
The Sons led after 11 minutes when Tom Walsh crossed for Gallagher to bundle the ball home.
United levelled on the restart when a Billy King hanging cross was powered home by Ralston.
The hosts reclaimed the lead just before the hour when Barr headed home from a David Wilson corner.
A topsy-turvy clash took another twist when United levelled again with 70 minutes played, Ralston’s driven cross being prodded in by McDonald.
The decisive contribution came at the other end when Dumbarton grabbed victory as another Wilson cross found centre-half Barr, who nodded in the winner.