Hart, who has held senior positions at Dixons, Abbey, Woolworths, Big W and ITM Communications, has been chief executive of Caffè Nero for the past five years, during which time the business has more than doubled in size.
Thorntons chairman John von Spreckelsen said: "Jonathan brings a wealth of relevant experience to Thorntons from the retail and consumer goods sectors.
"He has an impressive track record in successfully running a number of multi-channel businesses, including concessions, and a proven track record in the development of retail format strategies, as well as new product development and launches."