Anthony Stokes gave the Hoops the lead with an instinctive strike in the 20th minute when Well keeper Lee Hollis failed to collect the ball after charging from his line.
Neil Lennon's side had clearly shrugged off their midweek Champions League defeat to Barcelona and although Kris Commons missed two good chances before the break to extend their lead, he redeemed himself with a wonderful goal four minutes after the break.
Motherwell, unbeaten in their previous seven games, offered little in attack until late in the game when the points were already lost.
Stevie May and Dave Mackay had the Perth men two up at the break before Stevie MacLean embellished a fine passing display with a late double.
The result meant the Ayrshire outfit are the only side in the top flight still searching for their first win of the season.
The Saints victory also saw them leapfrog Killie, who slipped to 11th in the table.
Jon McGinn headed a 42nd-minute opener following a positive first-half display from the visitors and Paul McGowan capitalised on an uncharacteristic mistake by Hearts goalkeeper Jamie MacDonald to make it 2-0 in the 58th minute.
Hearts had gone into the game looking to further cut the gap created by their 15-point administration penalty but St Mirren picked the perfect moment to record their first victory of the season.
There was not much to pick between the teams but Quinn notched the winner seven minutes into the second half when the midfielder found the back of the net with a stunning strike.
The victory lifted Ross on to 10 points, four points behind fourth-placed Aberdeen.