Henry Sewell was born on 7 September 1807, the younger brother of William Sewell, and was educated at Hyde Abbey School, near Winchester, before qualifying as a solicitor and joining the family firm in 1826.
His brother William relied upon him for legal advice during the foundation of Radley, but this advice was often ignored – Henry was deeply concerned in June 1847 that the lease on the Radley Hall estate was still not signed despite Sewell and Singleton storming ahead with the inauguration of the school.
He became a member of the Canterbury Association, which planned to form an Anglican colony in New Zealand, and was apppointed their deputy chairman in 1851. In 1853, the Association was wound up, and he travelled to New Zealand to oversee the disposal of their assets. He would live in New Zealand for seventeen of the next twenty-three years (1853-56, 1859-66, and 1870-76), and became a prominent figure in early colonial politics, becoming the first premier in 1856. He later became treasurer (and effectively deputy premier), then headed the land registry and served several terms as attorney-general, before retiring to the United Kingdom in the late 1870s. His journals from this period provide one of the fullest accounts of persons and places in early Canterbury and the beginnings of self-government in New Zealand.