Gary MacKay-Stevens set up Stuart Armstrong to put United ahead at Tannadice before Jamie Hamill's penalty levelled the scores four minutes later after Mark Wilson had tripped Jamie Walker.
Brian Graham restored United's lead in the 49th when he headed home from a corner before MacKay-Stevens curling shot made it 3-1 in the 85th minute.
John Rankin's deflected shot made it 4-1 with the last kick of the match as United won their third straight league match and heaped pressure on the Jambos, who have now conceded 11 in their past two matches.
St. Mirren dominated the first half with Steven Thompson hitting the post.
However, Inverness improved after the break and Billy McKay and Marley Watkins combined to hit the woodwork as neither side could make the breakthrough.
Scott Vernon teed up Peter Pawlett to give 'the Dons' a 17th minute lead before Saints substitute Rory Fallon was sent off in the 70th minute just a minute after he had come on to replace Nigel Hasselbaink.
Aberdeen took advantage of their extra man with Niall McGinn running the length of the pitch to score a great solo goal in the 85th minute to seal the win.
Winger Chris Johnston popped up at the back post to score Killie's opener after just seven minutes.
However, County equalised when Stuart Kettlewell was well-placed to lash home Melvin de Leeuw's saved shot in the 33rd minute.
Boyd restored Killie's lead when he dinked the ball expertly into the top corner two minutes before the break, before he was denied by the woodwork twice in the second half.
Thistle had taken the lead through a wonderful goal from Kris Doolan in the 49th minute before Collins popped up to grab a point for the Easter Road side.