Wesfarmers owns Kmart, Target, Bunnings Warehouse and Australian supermarket chain Coles, among other businesses. The company has a sharemarket value of NZ$53 billion.
English said he looked forward to his role as a director.
Wesfarmers chairman Michael Chaney said English had an “outstanding record” in finance and policy development in New Zealand.
English became acting prime minister in December 2016 after Sir John Key resigned. Key has since joined the boards of Air New Zealand and ANZ. English resigned as leader of the National Party in February. Simon Bridges replaced him.