The retailer said that Thornton, who has worked at the company since 1966, would retire as a non-executive director on 1 September.
Thornton is the grandson of Joseph William Thornton, the group's founder. In 1982 he was appointed MD of the UK business. From 1987 to 1995 he also held the roles of chairman and CEO and retired as executive chairman in April 2004.
Also retiring as a non-executive director on 1 September is John Jackson, who has worked at the company since 1995. John von Spreckelsen, former Somerfield chairman, who was appointed chairman of Thorntons in June, said: "I am very sorry to be losing John Thornton and John Jackson after many years' invaluable service to the company. I would like to thank them for their advice and counsel since my appointment and wish them well for the future."
Meanwhile, the group has appointed Paul Wilkinson as a non-executive director. Wilkinson, who takes up his role with immediate effect, is a former executive chairman of RHM. He is currently a senior independent director for food and agri-business group IAWS.