The summer signing from Falkirk deftly headed home a David Barron cross in the 25th minute at Somerset Park.
Ayr competed well in midfield but lacked the physical presence at either end to cause a shock.
Home midfielder Dean Keenan was sent off in the final minute as Ayr pushed men forward, and Billy Mehmet secured the victory 60 seconds later.
Saints started brightly, Craig Samson pushing away Mehmet's 30-yard free-kick before Stephen McGinn placed another long-range effort just wide.
Ayr came into the game with Mark Roberts threatening to unlock the visitors' defence, and the striker had a glancing header saved by Paul Gallacher following Chris Aitken's near-post corner.
Tom Brighton, making his first start since his St Mirren debut more than 12 months ago, injected some life into their attack by bursting inside and firing a low drive that was turned past by Samson.
The resulting corner was cleared. But Barron quickly collected a throw-in and returned the ball to the penalty spot, where Higdon glanced the ball into the corner.
St Mirren held on comfortably until the break, Ryan Stevenson finding himself quickly closed down as he tried to turn and shoot following another Aitken corner.
Mehmet saw a free-kick in a dangerous position blocked just after the restart, before the visitors soaked up a period of home pressure.
St Mirren almost hit the hosts with a sucker punch as Mehmet created two chances in quick succession.
Dorman dragged a shot well wide when he was teed up inside the box, and Kenny Connolly did just enough to put off Higdon as he attacked a cross at the far post.
The Scottish Premier League side were now keeping their hosts pressed back, and Thomson - back in midfield following suspension - struck the bar in the 70th minute after winning the ball 20 yards out.
Higdon had a low drive saved from a similar distance - and although Ayr pushed forward, they failed to make the most of their set-pieces or break down a solid Saints defence.
Any hopes of a comeback disappeared when Keenan was sent off for a professional foul after grabbing hold of Mehmet when the Saints striker took the ball past him on the left wing, with Ayr heavily outnumbered.
Mehmet rubbed salt in the wounds in injury time when he slotted the ball home from 10 yards, after substitute Stephen O'Donnell's low cross was diverted into his path.