The Tannadice club savoured their first win of the new Scottish Premiership campaign by beating Saints 4-0 and now turn their attention to their tricky second-round tie away to the Scottish Championship side on Wednesday night.
And McNamara, in his first full season at the helm, believes his players have given themselves the ideal platform going into their clash with Dumbarton.
He said: "There were so many positives to take from Saturday's result against St Johnstone. Both the result and the performance was what we were looking for.
"I know the players are capable of that. Now we have to try and move on from there when we go to Dumbarton in the cup.
"A win like the one which we've just had always helps confidence, and the lads are feeling good after that.
"They're looking forward to the next game and it's one where we want to see a good performance and hopefully a positive result."
McNamara has had Dumbarton watched, so is taking every precaution to ensure there is no upset. Ian Murray's side beat Morton 3-1 on Saturday.
He said: "We've watched them so know they're a decent side. They had a very good result against Morton last weekend.
"We know we will be in for a very typical cup tie. It won't be easy and we realise that, so it's up to us to go about our business well."