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."
Meanwhile, Steven Thompson is not afraid to admit he is "visualising" Hampden glory every day ahead of St Mirren's Scottish Communities League Cup final against Hearts next weekend.
However, the 34-year-old former Rangers, Cardiff and Burnley striker is ready to give everything when he faces another of his former clubs, Dundee United, in their Clydesdale Bank Premier League clash in Paisley tomorrow.
As soon as that match is over, Thompson will return to some sports psychology techniques he has picked up over the years to get himself in the best frame of mind for the trip to the national stadium.
"We obviously have tomorrow's game against Dundee United and have to show a reaction to losing to Dundee during the week, to give belief and confidence back to the supporters for next weekend," he said.
"But there is no doubt you would be lying if you said you weren't thinking of next weekend. It has certainly been at the forefront of my mind for quite some time.
"I think about it on a daily basis. I don't mind talking about it. I am willing to admit that basically it is consuming most of my head space and has been for weeks and I am not scared to admit that.
"I have been visualising plenty of things and they are all positive.
"Visualising is an important part of preparation for sport and in my head I have got quite a few powerful visions.
"You pick up bits and pieces through the years. I have obviously been at a few clubs and sport psychologists have come in and out throughout during my career and talked to players.
"A lot of people were talking about visualisation during the summer at the Olympics and how it can improve your performance and it is a tool I try to use."
Asked if a last-minute winner at Hampden was his perfect scenario, Thompson smiled and replied: "I would take any minute, it doesn't need to be the last."
Boss Danny Lennon, though, insists the Dundee United match will not simply be a warm-up for the cup final.
"I won't be resting anyone," he said. "It is not a cup final preparation, it is all about getting three points.
"The players will have one eye on next Sunday but rest assured we will get the right application and right attitude come three o'clock on Saturday."
Lennon revealed that Lee Mair is facing a race against time to be fit for Hampden.
The Buddies defender picked up a stomach injury in the 2-1 SPL defeat by Dundee in midweek which is causing the Paisley boss "real concern."
Lennon revealed Jim Goodwin's calf complaint makes him "touch and go" for the match against Dundee United while Gary Teale is making good recovery from his abductor problem and Thompson will shrug off a tight groin.
Asked if any of his players could be struggling to make the final, Lennon said: "At this minute in time we have got real concern over Lee Mair. He is struggling with a stomach strain."