United had a man sent-off for the second game in succession, when Calum Butcher was dismissed after 20 minutes when he conceded a penalty.
Sean Dillon was short with a headed back pass and May latched on to the ball before he was pulled back by Butcher, and from the resulting spot-kick the Saints favourite confidently dispatched the ball past the keeper.
St Johnstone doubled their lead when United struggled to clear the danger from a David Weatherspoon shot and May (57) skipped past Dillon to beat Rado Cierzniak from a tight angle with a shot so powerful the ground-staff had to repair the net.
May completed his treble two minutes from time when he turned the ball in from close range from a low Gwion Edwards delivery.
Saints had started the game confidently and Edwards had a low drive well saved by keeper Cierzniak, before Frazer Wright sent a volley over the bar at a corner.
Following the opening goal Andrew Robertson cleared a Murray Davidson header off the line, while United created a half-chance which Ryan Dow planted against the side netting from a tight angle.
Davidson then twice came close just before the half hour as first Cierzniak blocked well from a low drive while a scrum of United players were required to deny him a second as the St Johnstone midfielder tried to scramble it home.
Right-back Mark Wilson thought he had levelled the match with a whipped free-kick that flew just wide of Alan Mannus' right-hand post, but it would be the last opportunity for United to get back into the game.