Marquardt moved into The Playboy Mansion in October 2002 and lived there as one of Hefner's partners until January 2009. She appeared in the first five seasons of The Girls Next Door alongside Kendra Wilkinson and Holly Madison.
She shared the below photo of the late publishing icon and herself along with a somber message.
"So sad but forever grateful for all the wonderful memories and amazing experiences," she wrote.
Hefner died of natural causes on Wednesday at his home surrounded by his loved ones.
Marquardt was the first of the reality star trio to speak on his passing.
"Thank you for being a revolutionary and changing so many people's lives, especially mine," McCarthy wrote. "I hope I made you proud."