A long-term adviser of M&S, Swannell will join the retailer as a non-executive director in October and will take up the chairmanship on 4 January when Sir Stuart Rose steps down.
Swannell spent more than 30 years in investment banking with Schroders/Citigroup. He is currently chairman of entertainment retailer HMV Group and non-exec for British Land and 3i Group.
Although his retail background is limited, Swannell's appointment was largely welcomed by the City, with analysts declaring him "a safe pair of hands" who would get on well with CEO Marc Bolland. "Robert has strong complementary skills and I am very much looking forward to working with him," said Bolland.