British colonizer and politician who served as the first premier of New Zealand (1856) after the colony had been granted responsible government.

As a solicitor in London he became secretary and deputy chairman of the Canterbury Association for the Colonisation of New Zealand and was sent out to New Zealand in 1852. He remained in the colony, opened a solicitor's office, and became a member of the House of Representatives in 1854.

Sewell became prime minister on May 7, 1856, but he resigned on May 13 because his ministry failed to obtain sufficient support in the House. He later served as colonial treasurer and commissioner of customs (1856–59), as attorney general (1861–62), and twice as minister of justice (1864–65, 1869–72). Toward the end of his career he clashed often with Prime Minister Sir Julius Vogel over Vogel's policy of heavy borrowing for development.