The magnificent Nadir Ciftci was at the heart of all their best work and a first-half brace for the Turk took his season's tally to seven.
"The first half was as good as we've played," said McNamara. "I thought we were great right from the start.
"We had good energy about us and there were so many good performances. It could have been a lot more than 2-0 at half-time - we had so many chances."
Youngsters Ryan Gauld and Chris Erskine were also on the scoresheet for the home side, but it was Ciftci who really caught the eye.
"He's getting better and better. He has so many attributes; his strength, awareness and finishing," said McNamara.
"Gauld had a couple of chances in the first half and was a bit unfortunate but his finish was fantastic. He is great to watch, with his ability and his vision.
"John Souttar is great coming out with the ball from the back and Andrew Robertson has been a revelation since he came here."
Under-fire St Mirren manager Danny Lennon added: "I've told the players not to dwell on this.
"This performance is very disappointing given what we had done in the previous three games.
"That's taking nothing away from Dundee United. They have some of the most
exciting young players in the country just now.
"We huffed and puffed at times and didn't work as hard at keeping the ball as we have done in recent weeks.
"We'll take this one on the chin and dust ourselves down ahead of the Ross County game next weekend. This was unlike us but it's important to move on."