The fifth-round tie at Tannadice provided the platform for Cameroon international Eric Djemba-Djemba to make his St Mirren debut and it was the Paisley side who threatened first.
Former United striker Steven Thompson exposed some sloppy defending when he chested down a long ball and volleyed straight at goalkeeper Radoslaw Cierzniak.
Tangerines captain Stuart Armstrong tested Kello moments later and United thought they were in on goal again when the skipper released Mackay-Steven but Darren McGregor's tackle suppressed the danger.
Ciftci and Gauld celebrate Dundee United's opener
However, St Mirren were not so robust when Ciftci raced away from Djemba-Djemba and centred for Gauld to apply the scoring touch.
The Buddies had fallen out of the game by that stage but an injection of pace from McLean changed the complexion of the first half.
The midfielder burst forward and got in between Paul Paton and Mark Wilson to draw a foul from the latter. And McLean could scarcely have been more composed as he passed the ball into the net from 12 yards.
Mackay-Steven was getting a lot of joy behind the St Mirren defence and only the prowess of Kello denied the winger a goal on the volley.
And the keeper made another terrific save after Ciftci had headed Armstrong's teasing delivery on target.
But Ciftci did find the net early in the second half, heading in Mackay-Steven's hanging cross.
McNamara's men were keen to put the tie to bed and Gauld's impressive shot on the turn was touched over by Kello.
Gary Teale replaced Djemba-Djemba and the former Scotland winger's free-kick delivery supplied St Mirren's best chance of a second leveller.
Thompson made the right connection with a powerful header but Cierzniak was equally alert to make the save.
McGregor fired wide following another set piece as time began to run out for the visitors.
United had one final effort, with Kello palming substitute Ryan Dow's shot on the run.
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