The eccentric comic suffered the medical incident at the Lone Wolf Comedy Club in Bicester last night (Thursday).
Paramedics were called to the venue in Oxfordshire but he was pronounced dead at the scene.
Reports say he had been taking medication for a heart condition.
Comedians are now paying tribute to the funnyman on social media, describing him as a “legend of the circuit”.
Actor and comedian Rufus Hound posted on Twitter: “Ian Cognito has died.
“That might not mean much to you if your knowledge of stand-up only extends to a screen but for anyone who ever sat down in a comedy club and saw him on a stage – this is a hard one.
“Puck grew up and now Puck is dead.
“We have lost one of the greats. S**t.”
Joe Bor wrote on Twitter: “A sad day for the comedy community.
“He was a legend of the circuit. Funny and unique. Incredible writer and performer.”
Mark Oliver said: “A sad day for comics today. The great Ian Cognito passed away last night.
“One the most charismatic performers and men (most of the time) I ever met.”
Brendan Burns tweeted: “He changed the face of this circuit single handedly. He was rallying against political correctness before it even had a name. I also love it when he pisses in my mouth.”
Cognito won the Time Out Award for Stand-up Comedy in 1999, and was compared to Lenny Bruce, Bill Hicks, and Jerry Sadowitz during his career.
But he never reached success mainstream media because, it is claimed, of his irascible reputation.
Despite this he continued to make a success on the comedy circuit and toured venues across the UK.
In an interview in 2012 the comic described his humour as “nasty” but “funny”.
He said: “Ultimately they should wanna be funny, then they’ll be liked. Simples.
“Nasty can be funny, so long as you don’t take it seriously. Best thing about getting old, a f**k you can not give.
“Anyway, no matter how nice you are, someone, somewhere thinks you’re a c***. Sorry to tell you that.
“Course in Jimmy Carr’s case someone somewhere thinks he’s alright.
“I didn’t do much comedy before I did comedy. Mainly tears in our house.
“For what it’s worth I see quite a few established comedians nicking my moves. They know who I am.”