With Steven Anderson dismissed after 38 minutes, the visitors were reduced to nine men with more than 20 minutes remaining after Chris Millar picked up a second booking.
As St Mirren threw everything at them to try to fashion a late winner, the St Johnstone defence stood resolute. The celebrations from their players and small band of supporters at full time were a sign of relief as much as of joy.
St Mirren's finishing was poor as they got increasingly desperate towards the end but when they did manage to find the target goalkeeper Alan Mannus was in inspired form.
When Mannus dived to parry John McGinn's shot in the fifth minute it set the tone for what was to follow, as he frustrated one St Mirren player after another.
He was not the only defensive stalwart; Frazer Wright was in the right place to hack an effort off the line, and the likes of Tam Scobbie and Callum Davidson also showed a willingness to put their bodies in the line of fire.
Manager Lomas watched it all from the back of the stand as he served the first of an eight-match touchline ban. That left his assistant Tommy Wright with the thankless task of trying to maintain order amid the chaos.
"Our players put their bodies on the line and worked extremely hard to keep it down to the one goal," he said. "I'm really proud of them all. But I wouldn't like to go through that every game. It's not a comfortable experience."
For St Mirren, it was a frustrating afternoon. The better side for much of the game, even when St Johnstone had 11 men on the pitch, they failed to make their superiority count.
The only consolation was that the draw at least extended their undefeated sequence to four matches. However, manager Danny Lennon said: "It's definitely two points dropped for us. We've got to be more ruthless in front of goal.
"You can't expect to give up opportunities like that in front of goal and still win a game of football. We did our fans a disservice today."
Given the rearguard action they were forced into in a frenetic finish, it was easy to forget that St Johnstone had actually been ahead. Their goal came after 15 minutes, Murray Davidson heading in from 10 yards after his namesake Callum swung in an enticing cross and Liam Craig nudged the ball in the younger Davidson's direction.
Anderson was sent off for bringing down Lewis Guy as the striker weaved his way towards goal. Mannus made a terrific stop from Kenny McLean's free-kick but it would get worse for St Johnstone midway through the second half when Millar upended the impressive McGinn and was shown a second yellow card.
A minute later, Lennon's men were level, Paul Dummett following up McLean's shot to slide in his first goal for the club and it was all St Mirren after that.
Gary Teale lashed over a shot, Mannus made another smart save to deny David van Zanten, McGinn's shot clipped the outside of the post, and Wright kicked Steven Thompson's effort off the line as St Johnstone bravely held on.