Lisburn Distillery's defeat means they will have to play Championship runners-up Newry over two legs with their Premiership place on the line.
Victory would have guaranteed the Whites' survival and they took the lead on 11 minutes when Gary Liggett shot in his 22nd goal of the season.
Alan Teggart equalised early in the second half and Gary McCutcheon put United in front with a penalty.
McCutcheon got another and Distillery's Jordan Hughes netted a late penalty.
Distillery will be at home to Newry in the first leg of the promotion/relegation play-off on Tuesday, 1 May, kick-off 1945 BST.
The second leg is at the Newry Showgrounds three days later.