Bethell was born at Kingston-upon-Hull, the son of William Froggatt Bethell. He joined the Royal Navy and served in the Egyptian war. In 1881 he was a lieutenant student at the Royal Naval College, Greenwich. He reached the rank of Commander and at the 1885 general election he was elected Member of Parliament (MP) for Holderness. In June 1899 he was authorized to assume the rank of captain. He fell out with his constituents over his hostility to the government's South African policy, and the Holderness Conservative Association in February 1900 voted that he had "alienated the confidence of the party which returned him at the last election, and that it is impossible to accord him their support in the future." He did not stand for the general election held in October that year. On 28 June 1919, he was appointed a deputy lieutenant of the East Riding of Yorkshire, but died towards the end of the year.