The prolific May was on target after 27 minutes in a rescheduled clash at Firhill, which came after the initial meeting in December was abandoned at half-time due to poor weather.
A fine run from David Wotherspoon provided May with the opportunity to grab the match's only goal and take his tally of Premiership goals this term to 16 - more than any other player in the top flight.
St. Johnstone also saw Gary McDonald have a goal ruled out for offside as they dominated the first half.
However, Partick improved after the interval and came close to an equaliser on several occasions.
Alan Archibald's men remain 11th in the table with 20 points from 23 games, while St. Johnstone have cemented sixth place in the table by moving four points clear of Hibernian.
