The Scottish Premiership side confirmed on Wednesday in a two-line statement that Kearney was to leave the club by mutual consent.
The Ballymoney man released his post, with the tag "A week is a long time in football..." soon after.
It read: "I'm gutted. But my time was up, that was crystal clear. I gave 100% commitment to the cause as I've done at any club I've been at as a player or manager.
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"I'm very proud of the job I did and the turnaround I made, just sad not to see it through."
Kearney paid tribute to his wife Lauren for her "relentless support", and also praised the St Mirren fans for the "faith and backing" they gave him.
In a two-line statement released by the the club they thanked the Ballymoney man for his spell at the club.
The statement said: "The Board can confirm that Oran Kearney has left the Club by mutual consent.
"The Club wish to thank Oran for his time in charge and wish him every success in the future."
What Kearney's future now holds is uncertain.
Former club Coleraine have already declared their interest in speaking with the 40-year-old.
Coleraine Chairman McKendry said it would be a "no brainer" to speak to his former manager if he was available again.
However, he acknowledges that there may be a number of clubs wishing to speak to the 40-year-old.
"It would be totally remiss for us not to talk to Oran," said McKendry on Monday evening.
"In my opinion though it will be difficult to bring him back as he may now have aspirations beyond the Irish League.
"I have no doubt there will be a number of clubs wishing to speak to him, but I can assure you we will definitely be having a conversation with him. It's a no-brainer really."
The Bannsiders currently without a new manager more than six weeks after the sacking of his successor Rodney McAree.
And Kearney has been heavily linked with a return to The Showgrounds since.
Kearney signed a three-year deal at St Mirren when he replaced Alan Stubbs in September.
He successfully saved the club from relegation as he guided Saints to a play-off victory against Dundee United last month.