Hearts ended a four-game winless run to stretch their lead in third as Stephen Robinson endured a miserable return to the Premiership with St Mirren.
Second half goals from substitute Ellis Simms and Cammy Devlin proved the difference but it was the 22nd minute sending off of Buddies playmaker Conor Ronan which did the real damage to Robinson and his side.
In front of the watching Scotland boss Steve Clarke, Barrie McKay impressed playing in behind Liam Boyce before moving wide left after the break and teeing up Simms for the all-important opener.
