The out-of-favour Aberdeen striker agreed a six-month loan deal with the Buddies 24 hours before the January transfer window shut as he sought regular action.
But that cost him a potential place in Derek McInnes' squad for their trophy decider with Inverness at Celtic Park.
With the final looming on Sunday, however, Magennis was honest enough to admit he has suffered some pangs of regret as his former Dons colleagues prepare for the Pittodrie outfit's first final in 14 years.
He said: "I'll be at the game and I do hope that Aberdeen win the cup.
"Of course I've got a few little regrets about missing it. If anyone tells you they would not regret missing out on a final they need shot.
"Being brutally honest, of course I'm going to have a couple of thoughts about the final and be disappointed not to be part of it."
But after starting just once for Aberdeen before switching to Paisley, Magennis insists it was time to move on.
"At the end of the day, I wasn't playing for Aberdeen and I had the chance to come here and play for St Mirren," he said.
"I didn't want to stick around for the final if it meant sitting in the crowd or only getting 10 minutes. As a footballer, if you experience a final, you want to play in it from the first minute till the last and then lift the cup.
"But I realised that wouldn't be the case with the form Adam Rooney is in. He is on fire and fully deserves his place in the team. I can't knock the gaffer for playing him.
"So it was an easy decision to come here, even if it meant missing out on a final."
While the Dons will be putting their final preparations in for Sunday's showdown, Magennis will be with his new Saints team-mates trying to battle away from the relegation play-off spot as they take on Dundee United at Tannadice.
But the Northern Irishman is certain the Buddies will survive.
He said: "If you look at the squad, you have Marian Kello who, for me, is one of the best keepers in the league, then the likes of Kenny McLean, Paul McGowan, John McGinn, Conor Newton, Steven Thompson - a variety of players who would be a big loss to this league. They are too good to go down.
"The club also has a fantastic fanbase, a great training ground and a great stadium. It is a very well-run club and we just need the performances to match what is going on in the background now."
- Sports & Recreation