United were 3-0 up before half-time as they proved much the hungrier side in the Tayside derby, and McNamara's men then cruised to their first win of the new Scottish Premiership campaign.
McNamara said: "It was a good win for us. We scored the goals at a good time and we took confidence from that.
"I'm delighted for David Goodwillie to get his first goal. I thought his all-round play was much better. He looked a threat.
"We were up and at them today. We didn't want to get into a fight with them and play for second balls. We wanted to get the ball down and play which is what we did well."