Celtic's most decorated player of all time, James Forrest, scored for a record-breaking 16th consecutive season in stoppage-time to make sure trophy day did not fall flat against a St Mirren who fell just short of European qualification.
With nearly the last kick of the ball, the veteran winger fired low past goalkeeper Zach Hemming to deny St Mirren a famous victory.
Jonah Ayunga's strike looked as if it would fire the Paisley side to only their second victory at Celtic Park in 38 visits.
It also looked as if it would seal a Conference League place, with St Mirren needing to better the result of a Dundee United side who trailed against Aberdeen.
However, United came from behind to lead before Forrest scored at the death deny Stephen Robinson's side the victory that their composed, disciplined performance seemed set to earn.
St Mirren's organisation and work-rate frustrated Celtic for most of the first half and, when their chance came, Ayunga fired hard and low into the bottom left-hand corner of the net when the home defence failed to clear a free-kick.
As if to underline the lack of intensity and quality from Celtic, Hemming was largely untroubled throughout, but was a relieved spectator when Daizen Maeda headed wildly over from substitute Forrest's fine cross.
It was not until the final few minutes when Hemming was called into action to deny late attempts from Johnny Kenny and Yang Hyun-Jun.
Forrest, Yang and Luke McCowan all came on for the last half hour for Celtic and, while the champions did improve slightly, St Mirren held firm with some terrific last-ditch defending necessary and also some composed play on the break.
Even had they won, they would not have been able to improve on sixth place thanks to United's win over Aberdeen.
For Celtic, how fitting that a modern-day great got the record-breaking goal before bringing the trophy out for their 13th title party in 14 years.