The Buddies' top six hopes suffered a serious setback in the Highlands as Joseph Hungo's penalty sealed a third straight defeat.
Stephen Robinson's side knew a win in Dingwall was crucial to keep up with the rest of the top half contenders.
It was a close encounter in the first half, with very few clear cut chances as both defences came out on top.
Alex Gogic's mistimed tackle proved crucial, as he felled Joseph Hungo in the box leaving the referee no other option but to point to the spot.
The Watford loanee dusted himself off to fire in the opener off the bar.
St Mirren tried to rally as the second half wore on, with Jordan Jones a constant thorn in the Staggies side.
But in the end they just couldn't force home an equaliser, consigning Robinson to third straight defeat since taking charge last month.
Robinson was forced into a change from the Celtic defeat with number one keeper Jak Alwnick ruled out after catching Covid-19.
Dean Lyness was brought in to replace him, with 17-year-old academy kid Kenzie Smith coming into the senior team for the first time as a substitute.
New Zealand international Alex Greive also came into the starting XI, with Robinson preferring him to Curtis Main.
Ross County started the game well, with flying wingers Regan Charles-Cooke and Joseph Hungbo looking dangerous.
The Buddies defence was dealing well with the threat though, with Joe Shaughnessy and Charles Dunne both pulling off some timely interceptions to keep the Staggies out.
Alex Gogic was impressive in the heart of the midfield, carrying the ball well and making a number of tackles to stop the hosts in their tracks.
The Saints were trying their best to hold onto possession and managed to do so as the half wore on.
They were struggling to cut the home defence open though. A Dunne pass over the top almost sent Greive in on goal but the ball skidded just out of the striker's reach.
Both defences were cancelling each other out, with Jordan Tillson's well struck volley over the bar the only real chance of note for the Highland side.
The Buddies finally cut the home side open just before half-time, with Alex Gogic breaking through and racing towards the Staggies' box.
The Cypriot almost had too many options and ended up choosing the wrong one, with Alan Power unable to control the pass through and Scott Tanser left unused in space on the left wing.
St Mirren made two changes at the break, with Curtis Main and Ethan Erhahon replacing Greive and Kiltie.
The second half couldn't have started much worse for the Saints. Alex Gogic got caught on the wrong side of his man and clumsily brought him down inside the box.
The Watford loanee stepped up to slam a superb penalty off the crossbar to break the deadlock.
The penalty aside the Buddies started promisingly, with Jones causing all sorts of problems down the left flank.
He burst to the byline before pulling back to Main, who drew a strong save from Ross Laidlaw down low at his near post.
Scott Tanser then saw a shot blocked as the Saints pressed hard for an equaliser.
The Staggies continued to pose a threat on the break, with Jordan White shooting wide of the post.
Lyness was then fortunate when he came to claim a corner and got nowhere near it, with the ball spinning past the post.
Eamonn Brophy was thrown on as Robinson looked to pose more of a threat in the final third.
Jones still looked the most likely to create or spark an equaliser though. He got on the end of a terrific Richard Tait cross but just couldn't direct it past Laidlaw.
St Mirren were desperate to avoid a third straight defeat and fought deep into extra-time to find a leveller.
They almost did when Jones again burst to the byline before throwing a cross to the back post.
A combination of Connor Randall and Alex Iacovitti managed to clear the ball away from Shaughnessy and secure all three points for the Staggies.
