Undoubtedly, United missed the creative spark of Barry Robson, who moved to Celtic in the transfer window.
But Craig Levein's side will be the happier with the replay back at Tannadice a week on Tuesday, their reward for a spirited defensive performance.
St Mirren played what little football there was and may have just deserved to edge the match but they could not find a way past inspired United keeper Lukasz Zaluska.
In a desperate opening 20 minutes the only goalmouth action saw Zaluska beat away Andy Dorman's 15-yard effort after some fine work by David Barron.
It was Saints that grabbed the game by the scruff of the neck and David van Zanten brought a fine stop from Zaluska with a low left-foot shot that the keeper did well to hold.
St Mirren should have gone in front when Andy Dorman managed to flick Mark Corcoran's pass wide of the target from only six yards.
It took another good save from Zaluska to keep United level after 38 minutes when Corcoran's fine flick from Ian Maxwell's cross looked netbound. However, Zaluska threw out his left hand and somehow clawed the ball round the post.
It was one-way traffic and Craig Dargo should have scored when the ball broke to him off Darren Dods but his right-foot shot slithered wide of the far post.
A fine passing move by the Saints ended with Billy Mehmet seeing his deflected effort drift wide as the home side strived for a goal.
They clearly did not want to endure a replay but try as they might they could not break the deadlock.
In the final five minutes van Zanten twisted and turned to create room for a shot but fired wide and Mehmet brought another save from Zaluska from the edge of the box.
There was no doubt St Mirren deserved to progress to the quarter-finals, but now they will have to win away from home if they are to reach the last eight.