Bertram was brought up on two small farms at Chalfont St. Giles (which his father owned for the purpose of sending his horses there to rest), where he developed his passion for horseback riding.
He continued working for the family firm until the outbreak of World War I, when he joined and served with the Royal Army Medical Corps, rising to the rank of captain.
After their father's death on 16 April 1938 aged 64, both Bernard and Cyril took over running the Bertram Mills Circus, sustaining its success until the early 1960s, when it was finally disbanded due to widespread television viewing.
Cyril Mills served with MI5 during World War II and was the spymaster who controlled Juan Pujol Garcia, codenamed "Garbo".
Bertram Mills was cremated (his father had been one of the first three undertakers to offer this service in London) and is buried with his wife at Chalfont St Giles in Buckinghamshire, England.