On-loan Oldham Athletic winger Dayton drove a shot into the bottom corner with six minutes played as the hosts failed to clear a corner at Firhill.
Ryan Stevenson was inches away from heading Partick level from Callum Booth's cross and they would press persistently after the interval.
But Alan Archibald's team, who had scored seven goals in their previous two outings, struggled to create clear openings as St Mirren belatedly closed out their maiden clean sheet of the season and marked the first match since manager Gary Teale's extended appointment until the end of the campaign with victory.
St Mirren are now 10th, a place below Partick, but they have played two games more than Motherwell in 11th, while bottom club Ross County have three matches in hand and are seven points behind Teale's men.
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