The Perth Saints were determined to inflict a defeat on their SPL-bound hosts and Steven Anderson was booked within two minutes.
The home side, nevertheless, made the early chances. Stewart Kean was just beaten to Billy Mehmet's cross by Simon Mensing, Charlie Adam headed a Kevin McGowne cross just wide and Kean fired over after persistent play by Craig Molloy.
Paul Sheerin replied with three low drives, forcing Tony Bullock to save and Ian Maxwell to hack the ball off the line but John Henry came closest to scoring, swerving a clever right-foot shot just wide from Jason Scotland's pass.
An early second-half header from Mehmet almost crept in but the visitors silenced the vociferous home support with a simple goal in the 67th minute. Matt Glennon launched a free-kick into the home box and the ball broke kindly for Steven Milne to knock it past replacement Chris Smith from nine yards.
Kean went close, Brian McGinty's shot-cum-cross struck the frame of the goal and Mehmet had a shot saved, but St. Johnstone saw out time efficiently, leaving St. Mirren to party - without Mr. McDonald.