Just how they'll react to their enthralling victory over Hearts in the Scottish Communities League Cup remains to be seen but their stunning 3-2 success has defined their season and Danny Lennon must now focus on putting together a side capable of competing in the top half of the table.
No fewer than 26 years had passed since St Mirren's last trophy success and when the pressure was on, I saw a number of players capable of handling the big occasion.
Jim Goodwin was a captain who led by example and his performance on the day was terrific. In front of him, ex-Celt Paul McGowan was also in inspired form with the experienced front pair of Steven Thomson and Gary Teale also playing leading roles.
Games against Celtic are always a huge test and I imagine the target for St Mirren in the last eight games of the season will be to climb the table if they can. With a 13-point lead over Dundee at the foot of the table, their SPL status is secure for another year but their League Cup triumph is a measure of how good they can be.
Not only did they overcome Hearts in a pressure final but along the way they also beat Aberdeen at Pittodrie on penalties before securing a memorable victory over Celtic in the semi-final at Hampden.
Victories like these show how effective St Mirren can be and like all managers, Danny Lennon continues to search for ways to ensure more consistency in results. He must know they're a better side than one sitting second bottom of the table.
That 3-2 semi-final defeat was a sore one for Neil Lennon. It's not often a treble is won in Scotland but with Rangers out of the picture, Celtic blew a glorious chance of winning all three domestic trophies.
With so many of Celtic's players away on international duty, Sunday's team will be a late call from Neil Lennon as he tries to assess how everyone looks on their return.
A few weeks ago, Celtic secured a Scottish Cup semi-final place with a 2-1 victory over St Mirren at the same ground but it's fair to say that on too many occasions this season, Celtic players have switched off and taken some games for granted.
Retaining the title is only a matter of weeks away and a bit of gloss can be put on a bizarre season by winning the Scottish Cup. But with the same domestic circumstances in place for next season, the big test for Neil Lennon will be to try and ensure his players are at the top of their game for the weeks building up to the new campaign when the qualifiers for the Champions League take place.
Thankfully, Celtic supporters had something to get their teeth into this season with qualification for the knockout phase providing the best memories for the fans.