The Tangerines take on Danny Lennon's men knowing they are approaching the business end of the season and could do with a win to boost their top-six hopes.
United ground out a hard-fought 2-1 Scottish Cup quarter-final victory over Dundee at Dens Park last weekend and McNamara would take more of the same in Paisley this weekend.
He said: "It's all about winning at this stage in the season.
"That is what it comes down to and we know that.
"It wasn't the best game to watch last Sunday but the main thing was getting into the next round.
"We'll have to be prepared to dig deep again against St Mirren as they are a difficult team to play against and we have had them watched.
"We always try to play football and hopefully we can do that but the bottom line is trying to get the three points."
United have aspirations to finish the second half of the season with a flurry and McNamara insists getting into the top half of the table is the immediate priority.
He said: "It's not long to go now until the split.
"There are just a few games left so we obviously want to take as many points as possible.
"We want to get into the top six and have a hard game against St Mirren at the weekend which we're concentrating on fully.
"They have players who like to play like ourselves so it should be an open game.
"We can go there with confidence as our form has been quite good of late but we know we'll have to be at our best.
"There are no easy games in the SPL and certainly not when you go away to play St Mirren who will be playing for cup final places."