Two minutes from the break Alan Mannus saved a poor spot-kick from the Kilmarnock midfielder after Saints left-back Callum Davidson was adjudged by referee Craig Charleston to have pulled down Gary Harkins in the box.
The former Hibernian player, who was booked for going in too hard for the rebound, compounded his error when he was dismissed in the 75th minute after picking up his second yellow card for simulation in the Saints box.
There was more drama to follow two minutes from time as Perth striker Sheridan, booked in the 70th minute for kicking the ball away, was handed his second yellow by Charleston, also for simulation inside the Killie box.
Aside from the two red cards which will dominate the headlines, neither side did enough to deserve to take all three points but Saints continue their quest for a European spot.
Kilmarnock face Celtic in the Scottish League Cup final at Hampden in a fortnight's time and they will have to show some marked improvement if they are to have a chance of silverware.
Killie manager Kenny Shiels made three changes to his side as he looked to beat Saints for the first time in three meetings.
Midfielder Liam Kelly returned from suspension while midfielder Danny Racchi and striker Dieter Van Tornhout also made the starting XI while James Dayton and Lee Johnson dropped to the bench with striker Paul Heffernan out altogether, reportedly injured.
Saints skipper Jody Morris was forced to drop out with a groin injury and he replaced by Chris Millar.
The visitors had the best of the first-half in terms of play.
There were less than two minutes played when midfielder Liam Craig tested Killie keeper Cammy Bell with a free-kick from 25 yards, following a foul on Saints striker Francisco Sandaza by Mo Sissoko.
Moments later, from Lee Croft's cross from the right, Kilmarnock defender Michael Nelson's clearance cannoned off Sheridan from close range and fortuitously ended up in the arms of Bell.
In the 16th minute, after a great cross from Croft found no takers in the Killie box, the home side almost took the lead when they threatened for the first time.
When Racchi whipped the ball into the box from the right, Van Tornhout crashed his header off the bar before Saints defender David McCracken cleared.
It was a let-off for the Perth side who still looked the more likely with another great ball in from the right, this time from stand-in skipper Dave Mackay, going a-begging to the frustration of the small band of travelling fans.
St Johnstone continued to be profligate and midfielder Liam Craig mis-kicked high and wide after Sandaza's chip found him alone at the back post and under no immediate pressure.
In the 37th minute Shiels escaped the Saints defence for the first time to cut the ball back across the six-yard box, forcing former Killie defender Frazer Wright to clear for a corner but that came to nothing.
Shiels then turned sinner after referee Charleston ruled that Davidson had tripped the clever Harkins in the box following a swift Killie break.
However, Mannus dived to his right to save the penalty, grabbing the ball at the second attempt, with Shiels picking up a booking in his bid to reach the rebound.
St Johnstone came out for the second-half looking to take advantage of their penalty escape.
In the 50th minute, though, Saints defender Wright did well to clear the ball over his own bar from two yards out, after Killie left-back Ben Gordon had raced on to a Harkins pass before driving across goal.
As play swung from end to end, at times erratically, it looked like one goal would be enough to win the match.
There were few clear-cut chances being created at either end but in the 75th minute Shiels was involved in more drama when he shown a second yellow card by referee Charleston for going to ground too easily in the box after going past MacKay, leaving the home side with 10 men.
The latter stages became irritable and fractured and two minutes from time, to cap a rather crazy day, Sheridan was shown a second yellow card for simulation inside the Killie box.
