United were favourites to progress and reach the last 16 but never really touched top form and the visitors took full advantage to make the fifth round for the first time in their history.
On a foggy afternoon United made a bright start and a Billy Robb free-kick saw a Paddy McCabe header go wide.
But Luncarty looked dangerous in took the lead in the 20th minute when striker Reg McLaren sent a low shot past Conor O’Boyle in the home goal.
Cumbernauld hit back and from a Simon Eeles cross McCabe shot over. Another Eeles ball should have caused danger in the Luncarty goalmouth but there were no takers.
Dylan McGuigan then got into the act with a great run and shot which well saved by Duncan Bruce. McGuigan then had another great effort but his lob over Bruce struck the bar and was scrambled clear.
United’s pressure however paid off in the 43rd minute when, following a Davie Dickson corner, McCabe left footed the ball past Bruce for the equaliser.
The Cumbernauld faithful were confident that their favourites would finish the job after the break but got a rude awakening when, following a poor United free-kick, the ball was cleared upfield to the dangerous McLaren who wasted no time in firing an unsaveable 20-yard rocket past O’Boyle.
McGuigan hit the bar for a second time, with a header from a Dickson corner, before, in the 70th minute, Luncarty’s Gerard Kelly was ordered off when he grounded McCabe in the box.
But Tony Stevenson’s penalty was brilliantly saved by Bruce and United’s misery was complete shortly afterwards when full back Darren Kelly’s free kick from 35 yards out sailed over O’Boyle and into the net.
United’s disaster day was further compounded when full back Alex Higgins’ bad tackle earned him a red card just before the final whistle.