Paul Hanratty, who has spent more than 30 years at the insurer, will continue in his role until Old Mutual's prelims are announced in March next year.
After that date he will step down from the board, but remain available to the group until the end of his 12-month notice period, which expires on 14 September 2016.
"I have been privileged to work at Old Mutual over a period when the company has developed exceptional businesses in both Africa and the UK," Hanratty said. "I would like to wish everyone at Old Mutual the best for the next chapter in its ongoing development."
Chief executive Julian Roberts said: "We are very grateful for his outstanding contribution to the success of the company over his long career with us and wish him every success for the future."