Charles O'Conor was born on January 22, 1804, in New York City, the son of Thomas and Margaret O'Connor and the great-grandson of Charles O'Conor who wrote a history of Ireland. In 1839-40, upon visiting Ireland and discovering that his ancestors spelled the name with one "n, " he adopted that form. Thomas O'Connor incurred the disapproval of the British authorities by engaging in the Irish rebellion of 1798 and found it wise to emigrate to New York in 1801. He earned a precarious living by writing editorials for the local press, and after his marriage to the daughter of Hugh O'Connor, and the birth of Charles, was able to give his son scarcely any educational advantages. His mother died in 1816.