McKenzie’s audacious first-half lob and Sammon’s header after the break, his first goal since returning to the club in January on loan from Hearts, earned the Rugby Park men just their second win in their last 12 matches.
St Johnstone were marking their 132nd anniversary, but marked the occasion with a sub-standard display.
It was Kilmarnock who began the match on the front foot and they created two opportunities before McKenzie’s 11th-minute opener.
With just four minutes on the clock captain Kris Boyd fired in a powerful free-kick that home goalkeeper Zander Clark had to beat the away.
Two minutes later Gary Dicker strode forward from midfield, but sent his 25-yard drive over the bar.
The breakthrough came from even further out, however.
Clark mis-kicked his clearance from a Steven Anderson pass back and it fell for McKenzie 40 yards out. The midfielder showed great composure and awareness to lob the ball back over the stranded Clark into the unguarded net.
Saints were rarely seen in attack, with forward moves breaking down in midfield before Freddie Woodman in the Kilmarnock goal had to be too concerned.
David Wotherspoon did have an opportunity with a free-kick in the 17th minute, but blasted his effort straight into the arms of Woodman.
Two minutes later Sammon saw his shot beaten away by Clark and in the 27th minute the home keeper was forced to turn away a Sean Longstaff shot before grabbing the midfielder’s header from the resulting cross.
Seven minutes from the interval St Johnstone finally threatened, but Steven MacLean’s header from a Wotherspoon centre looped over the bar and on to the roof of the net.
The home side knew they needed a response after half-time and they enjoyed more possession after the restart.
But they could not create any meaningful chances in attack and it was Kilmarnock who got the crucial next goal.
Jordan Jones swung in a cross from the left and Sammon rose to head down and on target from six yards out.
Clark in the Saints goal should have done better, but allowed the effort to squirm under his grasp and into the net to seal the home side’s fate.