A mistake from Saints keeper Jake Cunningham allowed Ryan Dow to put United in front and Jordan Elfverson doubled the lead after the break.
Sean Crighton - who has been in the first team squad on a few occasions - was sent off in the second half but Johnny Henry gave Saints a glimmer of hope when he scored with ten minutes left. However, in the final minute John Murray scored for United to seal the win.
The two sides will meet again in a few weeks time in the SFA Youth Cup, although this game will be played in Paisley.
For a more detailed match report and the thoughts of David Longwell, the club's head of youth development, visit the Official St. Mirren Website.