Leslie Jordan Was Headed to Set of 'Call Me Kat' When He Died

Leslie Jordan was about to do what he loved, acting and making people laugh, in his final moments -- he was on his way to set when he died behind the wheel ... TMZ has learned.

Sources close to the late actor tell us Leslie was scheduled to film for "Call Me Kat" Monday morning, but people working on the show became concerned when Leslie hadn't arrived more than an hour after his call time.

From there, our sources say there was a scramble to find Leslie -- and the cast and crew eventually discovered he'd been involved in a car accident.

TMZ broke the story ... Leslie was driving his BMW in Hollywood when investigators believe he suffered some sort of medical emergency, and crashed into the side of a building.

Sources close to the actor tell us he'd been complaining of shortness of breath the last 3 weeks, and was so concerned about the issue he'd actually had an appointment scheduled with a cardiologist.

Sadly, that was supposed to happen sometime in the coming week.

While it's unclear if that appointment would have discovered any underlying health issues for Leslie, investigators' current operating theory is he suffered a heart attack while driving.

Leslie's "Call Me Kat" costar Mayim Bialik posted a sweet tribute to the actor, writing, "They broke the mold when they made Leslie Jordan. He was a dear mentor and a beloved friend. I will miss him so much - it’s unimaginable that he’s gone. Rest well, sweet buddy."