The comedian, who cancelled his live tour due to “unforeseen family circumstances” in December, took to the stage at Blackpool Winter Gardens on Saturday evening.
The event was held to raise money for The Lily Foundation, which Kay previously described as “an inspiring charity” that helps to improve the lives of children with Mitochondrial Disease.
In footage of his brief appearance posted on Twitter, Kay, 44, told the audience: “Thank you very much for supporting the Lily Foundation raising money for mitochondrial disease. Very kind of you. I appreciate it. Enjoy the last ever episode of Car Share. Don’t tell anyone the end. Or tell anyone about any hedgehogs.”
Bowing as he left the stage, he added: “So thank you. Lots of love, enjoy, take care, bye-bye.”
The audience had gathered in Blackpool to see a sneak preview of the final ever episode of Car Share as well as an unscripted version.
Alison Moore, 45, said Kay appeared on stage after the interval between the episodes to a rapturous welcome.
“People were shouting out that they loved him, some stood up to clap,” she told the Press Association.
“He just said a few words for a couple of minutes, it was really good to see him.
“He mentioned the Lily Foundation, the fundraising, and good on him too - it shows what a decent kind of bloke he is.
“He seemed really fine, relaxed and genuinely touched by the reaction he got. He’s held in such high regard up this way. It was just really good to see him.”
A stream of fans took to Twitter to share their glee at the hit comic’s surprise appearance and praise the final episodes.
One said the Car Share unscripted episode was “pure genius” and added that it was “a joy to see Peter on stage tonight”.
“Absolutely loved it and the man himself stepping on stage abs blew us away #emotional,” another wrote.
Kay previously announced there would be three charity screenings of his popular BBC comedy programme at the Blackpool venue, each one airing a special finale to the series as well as an entirely improvised episode.
The second series of the Bafta-winning show ended on an emotional cliffhanger last year as John (Kay) failed to reciprocate the advances of his car share colleague Kayleigh (Sian Gibson).
Viewers were dismayed when he said there were no plans to pen a further series.
But the comic later announced special episodes - a special finale to reveal what happens next for the pair and the improvised Car Share: Unscripted.
Both hotly-anticipated shows, Unscripted and the finale, were filmed last year.
The screenings were initially announced on March 29 in what was the 44-year-old comedian’s first Twitter post since cancelling his live tour in December.
He said in a statement the cancellation was due to “unforeseen family circumstances”.
He added: “This decision has not been taken lightly and I’m sure you’ll understand my family must always come first.”