The red cards, however, played no part in an uneven contest where Killie barely laid a glove on St Johnstone.
Johnston conceded his troops had failed to perform to an acceptable standard.
He said: "It was really disappointing. The first goal was really poor and changes the game and to get caught by a long ball over the top was criminal.
"The fans are frustrated and rightly so because we've got to be better than that.
"That isn't good enough and we're all disappointed because we all want to win."
St Johnstone boss Tommy Wright felt his men were worthy winners and insisted the double dismissal had no impact on preceeedings.
The Northern Irishman said: "After about 10 minutes we were comfortable and switched the ball from side to side.
"Even if they had 11 men on the park I felt we deserved to win the game.
"I'd be unhappy if one of my players got done the way they were but referees are in an awkward position these days.
"The first goal in the SPFL is vital and you hope that makes them more vulnerable."
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- Nigel Hasselbaink
- Jackson Irvine
- Sean Clohessy