Inverness responded and Graeme Shinnie should have scored but failed to convert a decent chance. Doran then wasted another effort having been played in by Vincent.
It was St Mirren who came closest in the first half when Jim Goodwin curled a free kick over the wall and into a post.
Thistle were fastest out of the traps after half time and managed to take the lead just minutes after the restart when Vincent converted from the penalty spot following a handball offence from David van Zanten.
Their lead was then doubled when a mistake at the back left Doran with an easy finish.
McKay made it three when he flicked home a Doran cross before Van Zanten was sent off following a second booking for a foul on Ross Draper.