"Halfway through this year, I would have had my mind made up that I was going to retire," he says now, the big plunge taken after an 11-year association with the Dublin senior footballers, an All-Ireland medal and seven Leinsters amongst his stash.
"I wouldn't say it was an easy decision to make but I was very comfortable with it."
So far, himself, David Henry and Paul Casey are the only members of the All-Ireland winning panel to retire of their own volition, the former after 2011 ended and the latter, just days after Quinn's own announcement.
Reflecting now, he admits he had also considered walking away after the All-Ireland and calling it quits at the top.
Game time at a premium in an ultra-competitive Dublin forward line and the long-yearned-for medal nestling neatly in his back pocket, Mossy wasn't quite sure about going again this year.
As it happened, the extension of the September festivities almost dragged him into an 11th season like some form of celebratory riptide.
The group spent so much time together and the party bled into Christmas and before they knew it, Dublin were re-assembling for training and the notion of putting back-to-back All-Irelands began to turn their heads.
In his own head, Quinn figured fellas would be coming back with various levels of satisfaction and motivation and felt his own ambition, as a result of spending so much time on the sidelines the previous year, as strong as ever and that all told, he might just squeeze back into Gilroy's road map to greatness.
It didn't pan out like that and he made just one appearance as a sub in the Championship that never began for Dublin.
He doesn't like using the word "hunger," thinking it a cheap label which doesn't explain a whole lot but neither is he awash with reasons why Dublin never got going in 2012.
"If we're honest with ourselves, we hadn't performed all year. We were just getting by in our earlier matches and we didn't have one performance -- or even one half -- where it all really came together.
"We were going on the fact that we were just getting by and now we're down to the semi-final and we just need two big games and you think: 'there has to be a big performance in us,' but there wasn't.
"Hindsight is great but I don't really believe when people talk about hunger and how you prepare on the back of winning. We probably trained harder and put in more hours but there was a little bit of a fall-off.
"I just thought we were flat all year. I think we probably didn't find a balance in our play."
The theory that Dublin 'found their form' in the last 20 minutes against Mayo doesn't sit well with him either.
"Being honest, that was more about character and guys standing up and saying: 'we're ten points down here, I'm not accepting that,' he reckons.
"That just came from guys standing up and going back to basics and natural instincts rather than the gameplan clicking into gear or anything like that. And being honest, Mayo had two or three goal chances in that time and missed them."
The reasons are hard to find but he says the malaise was evident from within and painted brightly all over the team's results. The slog was certainly there but the effect wasn't so evident.
"We proved in the years previous that we need to be competitive in every game. I don't think we're good enough to just get by and turn it on further down the line and as it happened, we couldn't.
"A lot was made of the intensity of our A v B games in training the year we won it. This year, there wasn't one where it would have been at that intensity.
"We were trying to justify it and kept saying 'we're getting there' and we we're working our asses off and training hard and putting in the hours but for whatever reason, it just never got to the same level."
So as he made his way from the Dublin dressing-room to the team bus, Quinn was close to certain it would be his last such trip.
"There would have been a couple of guys that would have known the same, guys I would have been close to would have known it as well," he recalls.
He expected Pat Gilroy to stay on but that he himself would retire, and then at that meeting in DCU on the Tuesday after the Mayo match, his former team-mate announced he would be leaving after four years in charge.
"You start to wonder what the new man will bring and whether he would have a role for me," he admits.
So he met Jim Gavin, had a good discussion but nothing over the course of conversation swayed him sufficiently that his decision was going to change.
"You have to be honest with yourself," he says now.
He'll go back to the club and try and win another county title with Vincent's and there will be no part of him whatsoever that yearns for the winter slog and the early starts.
But there will be a moment when not being part of it all will feel weird and the reality of no longer being a Dublin player any more will sink in.
"The first league game in Croke Park," he says, "that's when it will hit me the most. It will be tough but I'm looking forward to it, I think. I'm looking forward to seeing how Jim sets it up, how they take shape and which young guys make an impact.
"I don't think I'll miss the six o'clock training or the January runs but being realistic, that's when it will hit me," Quinn concludes. "You're going into Croke Park and it will be the first time in more than ten years I'll be going in as a fan and not going in on the team bus."