In 1911 Bellairs was married to Charlotte, daughter of Colonel H. L. Pierson of Long Island, USA.
From 1913 he was a member of the London County Council as Municipal Reform Party member for Lewisham, resigning on 17 April 1915.
Carlyon Bellairs bequeathed his Barbados home, called Seabourne, as a research institute dedicated to the study of marine biology. McGill University in Montreal, Canada, founded Bellairs Research Institute in 1954 on the property. The institute as been expanded since, but the original house can still be seen close to shore and is often used as accommodation for visiting academics.