He said: “You don’t anticipate winning games like these in that manner, so we’re absolutely delighted.
“When they went to ten men it was probably the poorest we played all day, but we regrouped and capitalised on the space and deserved to win.”
He hailed striker Jai Quitongo on a fantastic performance after a lengthy spell on the sidelines last season.
Duffy added: “Jai’s only 19, he’s been injured for a long time and although if anything he’s even quicker since he came back, he’s maybe wanting to make too much of an impact too soon.
“He’s desperate to be successful, but today he showed his true quality, not just in terms of his running power, but his strength and his vision. But we had a lot of good performances and I’m delighted for the players and the supporters.”
The visitors should have been in-front with only six minutes on the clock when Lewis Morgan crossed for John Sutton but the Saints number nine headed just wide.
Morton had their first real effort when Ross Forbes curled a free kick just over the bar, much to the relief of Saints keeper Craig Samson.
The match heated up midway through the first half when the Ton were awarded a penalty kick, Quitongo bursting down the right flank and cutting into the box before he was brought down by Jack Baird.
Michael Tidser stepped up after a lengthy delay and dispatched the penalty, sending Samson the wrong way with a solid strike.
In the 38th minute the home side should have doubled their advantage when Quitongo crossed for Bob McHugh but his shot was brilliantly tipped over by Samson.
The visitors survived another scare a minute later when Thomas O’Ware headed just wide with a header from a Forbes corner.
And the Ton were punished for their missed chances when Saints drew level four minutes before the break, Cammy Smith striding towards the box before thundering a shot into the top corner.
The visitors were dealt a mortal blow four minutes into the second half when they were reduced to ten men, giving away another spot-kick in the process.
McHugh latched on to a loose ball and drove into the Saints box before Baird again was the culprit, bringing his man down to concede the penalty before being shown a straight-red card.
Tidser again took the spot-kick and restored his side’s lead, outwitting Samson for the second time.
The home side increased their lead in the 72nd minute when Quitongo again found space on the right and cut the ball back to McHugh who curled the ball home.
Five minutes later the score-line was becoming humiliating for the Buddies as McHugh fed the ball to Andy Murdoch who slid his effort into the corner of the net.
Jack Ross, the St Mirren manager, said: “I always speak frankly and I always let people in the dressing room know what I’m going to tell you guys – and I’ve told them that the reason we lost the game is that we were so poor defensively.
“The accountability will always fall upon me as a manager, but players don’t take the responsibility themselves often enough. As a back four, they needed to defend better.”