Saints started the better of the two sides, claiming the game's only goal inside six minutes.
Stephen Mallan's whipped corner evaded the entire Thistle defence before breaking to debut boy James Dayton who fired low beyond the helpless Paul Gallacher.
From there on in it appeared to be Thistle's game for the taking as they dominated possession but Gary Teale's men stood firm.
A dour second half followed with Saints' determination to thwart their hosts the most notable and telling feature of a match in which the result mattered more than the performance.
The reaction of the St Mirren players at the final whistle was evidence of how much the victory mattered to a team that, until now, had failed to record a clean sheet this season.
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