Second-half strikes from Chris Erskine, Radoslaw Fojut and substitute Blair Spittal condemned the men in black and white to their fourth straight Scottish Premiership loss, leaving them second-bottom in the standings.
Sloppy play in possession early on from Kenny McLean almost gifted the visitors an early lead when Erskine intercepted the ball from the St Mirren midfielder before firing straight at the onrushing Marion Kello.
Moments later and Gary Mackay-Steven came even closer to putting United in front when his powerful effort smashed off the right-hand post.
After those early scares the Buddies managed to stake a foothold in the match and, within the space of a couple of minutes, were unfortunate not to go ahead themselves.
Firstly John McGinn got on the end of an excellent McLean cross but his header was tipped over by the crossbar by Radoslaw Cierzniak in the United goal.
From the resultant McLean corner, Ross Caldwell was inches away from the getting vital touch from a Jason Naismith flicked header and the ball went wide of the right-hand post.
After surviving a short period of sustained pressure themselves, the Paisley outfit were denied a potential opener when United full-back Keith Watson managed a well-timed block from a Caldwell effort.
However, the remainder of the first half deteriorated with the standard of play slipping and a lack of creativity and imagination from both sides allowing for both defences to be on top.
Just as he did in the opening period, Watson came to the rescue for the men in tangerine minutes into the second half, deflecting a shot from Callum Ball out for a corner from just a couple of yards off the goal-line.
However, in the 47th minute United went ahead when unmarked Erskine raced clear of the home defence before smashing a low effort into the bottom-left corner of the net.
With the clock showing 68 minutes, United doubled their lead.
Centre-half Fojut rose highest to meet Mackay-Steven's corner and power a fine header into the net, leaving their hosts facing an uphill struggle.
Spittal was introduced with 10 minutes remaining in place of Erskine and almost had the perfect introduction after he picked the ball up in the St Mirren penalty area when Kello spilled a John Rankin shot into his path, but the substitute sliced his shot wide of goal.
But he did not need to wait much longer to get his name on the scoresheet, slamming high into the roof of the net in the 85th minute as the Saints crumbled.