Dundee United will go into 2018 five points adrift of Championship leaders St Mirren following this dismal defeat in Paisley.
Csaba Laszlo’s men should have taken the lead on 13 minutes but Paul McMullan’s poor penalty was saved by home keeper Craig Samson.
Having squandered that wonderful opportunity, United fell behind to a fine strike from Celtic-bound Lewis Morgan just before the hour mark.
Morgan added his second – almost a carbon copy of the first – on 78 minutes to ensure that the Tannadice team will be chasing rather than leading going into the new year, albeit they have a game in hand on the Buddies.
In truth, United never found the fluency of previous weeks and could not argue with the final result.
The Tangerines, backed by 1,246 fans, made just the one enforced change to their starting line-up from the win over Livingston last Saturday.
As expected, skipper Willo Flood came back in to replace the injured Fraser Fyvie, with the playmaker’s season over because of cruciate ligament damage. There were places on the bench for frontman James Keatings and young midfielder Kieran Inglis.
The game had been in some doubt because of the snow that fell on Paisley earlier in the day but the pitch looked fine as the action got under way.
McMullan raced up the right flank for United with two minutes of the clock but his cross took a deflection and was gathered by Samson.
The first decent attempt on goal was from the home team on nine minutes, with United keeper Harry Lewis having to look smart to save a low 20-yard drive from Saints skipper Stephen McGinn.
A Scott Fraser cross caused panic at the other end and eventually led to a penalty for the Tannadice men with an unlucky 13 minutes on the clock.
After the ball bobbed about the box, Ian McShane was penalised for a foul on Mark Durnan but when McMullan took the kick he placed it at a perfect height for keeper Samson and the score stayed level.
United’s Scott McDonald, who had been booked earlier for a foul on McGinn, had a great chance to open the scoring on 41 minutes when he got his head to a Fraser cross but didn’t have enough power to trouble Samson.
Just a minute later, Sam Stanton was gifted possession by St Mirren’s Liam Smith and squared to Fraser, who hooked his left-foot shot the wrong side of the post.
Morgan opened the second half for the Buddies with a shot that flew wide of the United goal then McShane did the same a minute later.
There was concern for the Tangerines on 50 minutes when Flood stayed down on the ground near his own goal. However, the captain soldiered on after treatment to his leg.
It would need something special to break the stalemate and that something special duly arrived on 59 minutes. With United’s players standing back to admire him, Morgan cut in from the left until in a perfect position just outside the box. He then fired in a a rocket shot that gave Lewis no chance to make it 1-0 to the hosts.
After much debate between himself and the coaching staff, Flood finally conceded defeat to his injury and was replaced by Matty Smith.
Keatings came on for McMullan on 71 minutes then an injured-looking Fraser was replaced by Scott Allardice five minutes later.
The match then looked well an truly beyond United when Morgan made it 2-0 on 79 minutes after Durnan conceded a freekick by fouling Cammy Smith.
It was a familiar story, with Morgan picking up possession, making his way in from the left and smashing a shot low from outside the box beyond Lewis’s outstretched left hand.
There was still time left for keeper Lewis to prevent a third goal by saving with his legs from Cammy Smith after the former United striker ran clean through on goal.
St Mirren: Samson, Demetriou, McGinn, Magennis, Morgan (O’Keefe 90+3), C. Smith (MacPherson 90), Baird, McShane, Mullen (Sutton 89), Davis, L. Smith. Subs not used: Stewart, Buchanan, MacKenzie, Duffy.
Dundee United: Lewis, Scobbie, Durnan, Quinn, McMullan (Keatings 71), McDonald, Fraser (Allardice 76), King, Stanton, Flood (M. Smith 63), Robson. Subs not used: Deniz, Appere, Chalmers, Inglis.
Referee: Don Robertson.