Lee Erwin blasted home a pair of emphatic left-footed strikes set up by Lionel Ainsworth to give the hosts a clear advantage and the game was virtually over as a contest when Scott McDonald converted from close range on the stroke of half-time.
John Sutton got in on the act with his own double during the second half, concluding a damaging evening for Gary Teale's St Mirren, who are now 10 points behind Motherwell in 11th following a fourth consecutive loss.
Ross County are three points better off than Motherwell in 10th after securing a vital victory of their own, 1-0 at home to St Johnstone.
A tense encounter was settled when Liam Boyce headed home Jamie Reckord's cross six minutes from time, extending County's remarkable resurgence to seven wins in an eight-match unbeaten run.
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