United were unable to find a way past the Buddies defence and finished the game with 10 men after John Rankin saw red two minutes from time.
McNamara, whose side missed the chance to move into the top six, said: "I thought that in the first half, we were a bit negative with our play - we'd get so far, go back to goal and it ends up at our goalline - but in the second half I thought we did better.
"Keith Watson got forward well down the right hand side and put some good balls in for Stuart Armstrong to get on the end; I thought that on another day we could have got a goal.
"I'm disappointed we didn't get the three points, as those were two extra points we wanted to go and win to get into the top six."
McNamara will review footage of Rankin's sending off before deciding whether or not to lodge an appeal.
He added: "I told the referee at the end that we'll take a look at it first; the way the lad turned into it I think made it look worse."