Rowan Vine netted the only goal at McDiarmid Park from close range in the 52nd minute after Murray Davidson had bundled Mehdi Abeid's corner towards goal.
Substitute Gregory Tade, twice, and Frazer Wright headed against the frame of the goal afterwards as the home side looked to put the game out of St Mirren's reach.
But Lomas was content with the narrow victory after his players recovered well from the exertions of their 1-1 midweek draw with Celtic, when substitute Nigel Hasselbaink earned Saints their point.
The Perth Saints moved just one point off second place and gave themselves some breathing space in the increasingly tight battle for a top-half place in the SPL.
Wednesday's trip to Dundee to face their local rivals, who appointed John Brown as interim manager at the weekend, is the second of three consecutive midweek games as Saints play six matches in a 23-day period that could define their season.
Midfielder Davidson came back from injury and left-back Callum Davidson returned to the team on Saturday, and Lomas admits he will need his full squad firing on all cylinders.
Lomas said: "The next game is critical. Dundee took on a new manager and that in itself will provide a difficult prospect, and it's a derby.
"I said to the lads we have got to prepare right. We then have another tough away game against Hearts and have Kilmarnock after that.
"Hopefully we'll get a couple back. Chris Millar is not far away and Dave Mackay was on the bench yesterday, we just didn't want to risk him.
"It's definitely a squad game and Tam Scobbie was unlucky to miss out. Hopefully we are getting a few back at the right time."
St Mirren manager Danny Lennon is hoping winger Gary Teale recovers from a stomach muscle strain to face Hearts in Paisley on Wednesday night after his side lacked the final ball to hurt their hosts.
The Buddies, and Esmael Goncalves in particular, looked the livelier team in a scrappy first half, but they struggled to carve out a chance when they were chasing the game.
Both St Mirren and Hearts are desperate for victory on Wednesday to revive their flagging top-six hopes and Lennon admits it is as much of a cup final as their March meeting with the Edinburgh side at Hampden in the Scottish Communities League Cup.
Lennon said: "I said to the players prior to the game that it was still very much in our hands and that prior to the cup final on the 17th we have five cup finals.
"We need to produce top performances and make sure we pick up victories.
"That's one out the road but we will be taking the next four and trying to give ourselves every opportunity.
"It's disappointing. That's two out of three games where we are playing on difficult pitches and totally going against our beliefs, but we have got to be intelligent and adapt to that."