The former Partick Thistle manager was involved in a heated touchline spat with opposite number Tommy Wright during Saints' 1-0 win at Tannadice back in March.
The exchange resulted in the pair being sent to the stands and Wright accusing McNamara of being 'disrespectful to his side'.
But speaking ahead of the end-of-season showpiece at Celtic Park, McNamara, who won the trophy three times as a player with Celtic, said: "This is a cup final and previous games, and everything that has happened in them, go out the window.
"It is not about revenge it is about winning silverware for this group of players who have put in so much work and effort throughout the season and it would be good for them if they can get over the line."
Asked if winning the trophy would be the greatest achievement of his career, McNamara told Sky Sports News: "You cannot compare playing to managing.
"People keep asking me that, but it's a different career now. unfortunately I'd still love to be playing. I prefer that to be standing on the side of the park but this is the next best thing. This is for the group of players, it's about them, about them being part of the history of the club, not about me.
"As a manager it's a bit more nerve-racking. As a player you just concentrate on your own job in hand and make sure you are in the starting XI.
"But as a manager you are making sure everything is properly organised. You have to pick the team and the subs, and you have to let people down who are not involved. There's a bit of heartache to come but that's part and parcel of the job."
And McNamara played down suggestions he is looking to further his career in England after being linked with the managerial vacancy at Championship side Blackpool.
"That proved no distraction to me at all," he said. "Nothing would take away from the preparations for the final. For me it was flattering to be linked with Blackpool but my focus is all on Saturday.
"You never know with football. You see the crazy goings on this week in Scottish football with managers losing their jobs and my sympathies go out to Danny Lennon and Gary Locke who have done fantastic jobs.
"So you never get carried away with wins and speculation. Just let other things take care of themselves."
Watch St Johnstone take on Dundee United this Saturday live from Celtic Park from 2.30pm on Sky Sports 1