An equalising header from Liam Craig in the third minute of stoppage time salvaged a point for St Johnstone.
The draw ensured St Johnstone will finish in the Scottish Premier league's top six after teenage midfielder Ryan Gauld scored his first league goal for Dundee United on Monday.
Gauld, 17, struck in the 23rd minute, running onto a fine Mark Millar through ball before coolly slotting home past St Johnstone keeper Alan Mannus.
But the visitors were reduced to 10 men in the second half when Stuart Armstrong received a second booking.
United had Radoslaw Cierzniak to thank for a share of the spoils, the Polish goalkeeper producing a string of fine saves.
The closest St Johnstone came to an equaliser before Craig's dramatic late header was in the 63rd minute when Steven MacLean beat Cierzniak with a looping header which rebounded back off the post.
After keeping out Murray Davidson's low shot, Cierzniak was finally beaten when substitute Mehdi Abeid picked out Craig from a corner.
United now require other results to go their way if they are to claim a top-six place.
Jackie McNamara's men are level on points with seventh-place Aberdeen ahead of their meeting on Saturday.
However Kilmarnock, who also sit on 41 points, have a game in hand against St Mirren on Wednesday before the final round of pre-split fixtures.