Argentine midfielder Franco Miranda marked a dream debut with the long-awaited opening strike and Mark Corcoran doubled their lead early in the second half.
Inverness quickly pulled one back through Don Cowie and battled all the way to the final whistle, but, after three straight defeats, they are stuck at the foot of the table, playing well but simply not getting results.
Seven days after losing two late goals at Motherwell, the visitors conceded early this week despite dominating the opening 19 minutes.
Graham Bayne, John Rankin and recent signing Marius Niculae had all gone close by the time St Mirren's new boy Miranda put them ahead from their first serious attack.
Skipper Hugh Murray played the ball forward, Miranda controlled it with his chest before sweeping it first-time past goalkeeper Michael Fraser with his right foot.
An entertaining game became even more open as play raged from end to end.
Niculae tested Chris Smith from distance and Corcoran forced Fraser into action, but Inverness still looked the stronger team and Smith needed to make two excellent saves to deny Cowie and Bayne.
There were no changes in personnel at half-time yet the contest suddenly had a different feel to it, as St Mirren kept a subdued Inverness side pinned back in their own half.
It took a second home goal to wake the Highlanders up and this came in the 56th minute when Stewart Kean reached the by-line and crossed for Corcoran to score from close range.
Caley were stung by not only the goal, but the prospect of slipping to the bottom of the table and they promptly pulled a goal back.
Niculae found space on the left and his low cross was slammed home at the back post by Cowie.
Rankin also got his second wind and almost equalised with a left-foot effort 11 minutes from the end, but the Buddies held on for a valuable three points.