Spokeswoman Kerri Richardson says Mike Haydon passed away Sunday afternoon. The cause of Haydon's death was uncertain but thought to be a massive heart attack.
Haydon became Beshear's chief of staff in 2010 after Adam Edelen resigned from the position. Before that, Haydon, who also was once the mayor of Springfield, served as Beshear's legislative director.
Warren County state representative Jody Richards, who worked with Haydon for decades in Frankfort, told WKU Public radio he was shocked and saddened by his sudden death.
"Mike knew state government inside and out," Richards said, "this is just a terrible blow."
House Minority Floor Leader Jeff Hoover said Haydon, a WKU graduate, was a "very good friend" and will be missed greatly.