So poor were the home side that it was a surprise the Buddies did not win by a far greater margin.
The visitors played with spirit and determination as they finally chalked up their first victory against the Tangerines, who had beaten them four times previously this year.
The visitors were out of the traps from the kick-off, but it was United that created the first real chances, putting two together in quick succession. Firstly, an excellent Morgaro Gomis strike was followed by a sweet Noel Hunt effort. But United were hit with two body blows after 20 minutes.
Hunt was given his marching orders, his second red card of the season, for a vicious kick on the ground to Kirk Broadfoot's facem and then, two minutes later, Hugh Murray scored a precious goal, hammering home a poor defensive clearance from Andy Milne's free-kick.
United were rattled as the Buddies completely dominated the half. John Potter was unlucky with a cracking 25-yard shot which zipped over the bar in the 37th minute. But, despite the visitors' pressure, there was little to trouble United keeper Derek Stillie.
Mark Kerr woke Chris Smith up with an edge-of-box pot-shot, which the keeper held well. But the game continued very much in the same vein as the first half with the Saints completely in the ascendancy.
Stillie did well to clasp a rasping cross from Mark Corcoran, then Porter flicked over a corner as the Tangerines defence spectated.
It was no surprise that, with the pressure building, Christian Kalvenes brought Broadfoot down in the box. It would have certainly won the game for the visitors but Stillie produced a magnificent save to push the ball around the post.
However, it was game over in the 88th minute when man-of-the-match Broadfoot, who is heading out of the door to Rangers in the summer, ghosted in to meet Corcoran's corner and nod down into the net from six yards.