Match ends, Celtic 1, St. Mirren 2.
St Mirren tormented the foundering Scottish Premiership champions to deservedly claim a first away win over Celtic in 31 years.
Kristian Dennis and Ilkay Durmus struck in the first half, either side of an Odsonne Edouard reply, as Jim Goodwin's men surged into the top six.
It ended St Mirren's eight-year wait to beat Celtic, and is their first league win over the Glasgow club since 2010.
Celtic's latest sobering setback leaves them 23 points behind leaders Rangers.
Neil Lennon's men have two games in hand on their Old Firm rivals, but their season continues to unravel at an alarming rate, with one win in their last six games.
St Mirren have responded to last weekend's League Cup semi-final heartbreak against Livingston by securing successive victories.
The day after Celtic announced chief executive Peter Lawwell will leave at the end of the season, the club's rudderless predicament was further underlined by an insipid performance.
St Mirren's previous league win in this fixture - a 4-0 trouncing in Paisley - spelled the end for Celtic manager Tony Mowbray.
Current incumbent Lennon continues in the job, but he was tactically outmanoeuvred by Goodwin as St Mirren exploited the hosts' vulnerabilities.
St Mirren's last 11 trips to Celtic Park had yielded a total of two goals. They reached that mark this time inside 37 minutes as Goodwin carried out his pre-match pledge to attack.
When Celtic's high-intensity start quickly fizzled out, St Mirren pounced. Dennis provided the breakthrough when he played the ball wide to Dylan Connolly then collected the return pass 10 yards out and crashed a shot beyond Scott Bain.
The hosts had offered nothing going forward, but hauled themselves level just after the half hour. Callum McGregor burst into the box and supplied Edouard to send a scuffed finish into the far corner of the net.
It was harsh on St Mirren, yet they responded within minutes. Not for the first time this season, Shane Duffy was found wanting. The centre-back failed to deal with a routine cross from Kyle McAllister as Durmus superbly took the ball down and dispatched his first league goal of the season.
Celtic's attempted comeback was fruitless as the visitors diligently protected their lead without much stress. A succession of efforts - mainly from distance - flew off target from Edouard, Duffy, Ryan Christie and Tom Rogic.
St Mirren goalkeeper Jak Alnwick did not have a serious save to make until injury time when he clawed away a Duffy header at the near post as the Paisley side were able to celebrate a landmark victory - their first at Celtic Park since April 1990.
Man of the match - Jamie McGrath
What did we learn?
Celtic's malfunctions are no longer a surprise.
The team are a shadow of the dominant force of recent years, and their confidence looks completely shot.
In contrast, Goodwin's stock is on the rise, with St Mirren having completed an Old Firm double by adding Celtic's scalp to that of Rangers this season.
Even with strikers Eamonn Brophy and Job Obika dropping to the bench from the team who hammered Dundee United in midweek, Goodwin's side were full of attacking verve and excellently organised. This performance will live long in St Mirren history.
What did they say?
Celtic manager Neil Lennon: "It was a poor performance, a poor result. We expect to build a crescendo of pressure and break teams down but that's been missing this season.
"The lack of belief and lack of quality - it's not just individuals, it's a collective, and it's symptomatic of the season.
"It's a low point in terms of the performance and the result - but I'll keep working away trying my best."
St Mirren manager Jim Goodwin: "It's an incredible afternoon for the players. We really fancied ourselves coming here today. The system we played was attacking and the boys bought into it brilliantly.
"We deserved the win, it wasn't a case of Jak Alnwick making a number of great saves. We were hanging on in the last 15 minutes but that was purely down to fatigue."
St Mirren begin a run of three successive home games with the visit of Hibernian on Tuesday (18:00 GMT), while Celtic are also in Premiership action the same night away to Kilmarnock (19:45).
- 6BittonSubstituted forWelshat 45'minutes
- 12SoroSubstituted forChristieat 63'minutes
- 14TurnbullSubstituted forJohnstonat 81'minutes
- 27ElyounoussiSubstituted forAjetiat 62'minutes
- 9GriffithsSubstituted forRogicat 45'minutes
- 21ConnollySubstituted forTaitat 85'minutes
- 14MacPhersonSubstituted forObikaat 74'minutes
- 10McAllisterSubstituted forDoyle-Hayesat 62'minutes
- 20DennisSubstituted forBrophyat 62'minutes
- Bobby Madden
- Shots on Target
Second Half ends, Celtic 1, St. Mirren 2.
Attempt missed. Jonathan Obika (St. Mirren) right footed shot from the right side of the box is high and wide to the right. Assisted by Eamonn Brophy with a headed pass.
Attempt missed. Shane Duffy (Celtic) right footed shot from the right side of the six yard box is close, but misses to the right.
Attempt saved. Shane Duffy (Celtic) header from the centre of the box is saved in the bottom right corner. Assisted by Greg Taylor with a cross.
Foul by Odsonne Edouard (Celtic).
Jak Alnwick (St. Mirren) wins a free kick in the defensive half.
Michael Johnston (Celtic) wins a free kick in the attacking half.
Foul by Jake Doyle-Hayes (St. Mirren).
Attempt saved. Michael Johnston (Celtic) right footed shot from the centre of the box is saved in the centre of the goal.
Attempt blocked. Odsonne Edouard (Celtic) right footed shot from outside the box is blocked.
Tomas Rogic (Celtic) wins a free kick in the attacking half.
Foul by Ethan Erhahon (St. Mirren).
Attempt blocked. Michael Johnston (Celtic) right footed shot from outside the box is blocked. Assisted by Albian Ajeti.
Substitution, St. Mirren. Richard Tait replaces Dylan Connolly.
Foul by Ryan Christie (Celtic).
Dylan Connolly (St. Mirren) wins a free kick in the defensive half.
Attempt blocked. Jonathan Obika (St. Mirren) header from the centre of the box is blocked. Assisted by Jamie McGrath with a cross.
Corner, St. Mirren. Conceded by Greg Taylor.
Attempt blocked. Dylan Connolly (St. Mirren) right footed shot from outside the box is blocked. Assisted by Ethan Erhahon.