The struggling chocolatier has been on the hunt for a "retail savvy" CEO since the departure of Mike Davies earlier this year.
Hart has been CEO of Caffè Nero for the past five years. He also has more than 25 years' experience in retail and consumer goods, at Abbey, Woolworths, Big W, ITM Communications and Dixons Group. Since he took up the reins at Caffè Nero, the business has more than doubled in size, with 400 stores.
"Jonathan brings a wealth of experience from the retail and consumer goods sectors," said Thorntons chairman John von Spreckelsen. "He has an impressive track record in running a number of multi-channel businesses, and a proven record in the development of retail format strategies, as well as NPD and launches."