Goals from Steven Anderson – a former United trainee – and Steve MacLean proved the difference to compound a frustrating final for Jackie McNamara's men, who were twice denied by the woodwork.
A spectacular save from Radoslaw Cierzniak kept out a rasping James Dunne volley with less than 15 minutes played before the post came to St Johnstone's rescue at the other end.
Andy Robertson's whipped cross from the left flank caused panic in the defence and Ryan Dow reacted quickest with an instinctive flick that cannoned off the post and rolled back across the goalmouth before being hacked clear.
And that moment of good fortune proved crucial as Anderson headed his side into the lead right on the stroke of half-time.
Cierzniak flapped and missed after he advanced off his line in an attempt to clear David Wotherspoon's corner and Anderson was lurking at the back post to direct a looping header into the top corner.
The frame of the goal again denied United after the restart. Nadir Ciftci curled a free-kick up and over the wall only for the ball to rebound off the crossbar and a thankful Alan Mannus fell fortunately onto it.
Stevie May was booked for bundling in the ball home from close range with his hand as St Johnstone pushed for a second that did arrive six minutes from time courtesy of MacLean to spark wild celebrations amongst the Perth club's fans.
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