United looked assured with some slick passing movements and Barry Robson put them ahead with a smart left-footed drive from 22 yards following a good Mark Kerr pass.
Saints sought the equaliser with energy and skill. Garry Brady's curling effort was well saved by Derek Stillie, John Sutton drove an angular shot past after holding off Christian Kalvenes and John Potter headed a Simon Lappin cross wide.
Chris Smith did well to keep out free-kicks from Robson and Craig Conway but United were gifted a second when Lappin and his defenders made a terrible hash of clearing a relatively unthreatening attack and Stuart Duff stepped in to rifle the ball home from a few yards.
David Van Zanten shot inches wide and Robson superbly defended a Lappin free-kick as Saints tried to recover from their disastrous defending.
Garry Brady pulled one back just after the break, sweeping in a Stewart Kean pass from the angle of the box. A Kean thunderbolt then narrowly missed a post, but Saints blew their comeback bid when Smith miss-kicked van Zanten's pass-back and Kirk Broadfoot failed to clear the ball away, allowing Robson to curl in number three.
Collin Samuel should have made it four when Duff sent him through but Smith made some amends by deflecting the shot with his foot, while Robson was sent off on 84 minutes after receiving two quick yellow cards.