The Scotland Under-21 international netted from eight yards after 27 minutes, finishing off some terrific work by David Wotherspoon to confirm a 1-0 win at Firhill.
Saints dominated the first half before Thistle responded with spirited display after the break.
But the Jags lacked the quality of final ball to really trouble Saints and Tommy Wright's men clung on to claim the win which took them to within a point of fifth-placed Dundee United.
The match - rearranged after the original fixture on December 14 was abandoned due to a waterlogged pitch - pitted a Partick side yet to win a home league fixture against a Saints line-up whose only previous away win outside of cup duty came against the whipping boys of Hearts three months ago.
Wright named Steven Anderson in his line-up side after appealing the defender's weekend sending-off against Hearts.
But goalkeeper Alan Mannus had to accept his punishment following the dismissal he received for clashing with the Jambos' Ryan Stevenson, as his place was taken by 41-year-old Steve Banks.
Jags boss Alan Archibald also had suspension concerns with Aaron Taylor-Sinclair - sent off against Kilmarnock on Saturday - and Stuart Bannigan both banned, meaning Jordan McMillan was joined in the Thistle starting line-up by the returning Chris Erskine following his loan move back to Firhill from Dundee United.
But May's goal just before the half-hour mark after Wotherspoon slammed into the box proved to be enough to claim the points as Thistle missed out on the chance to climb out of the relegation play-off spot.