In 1852 he entered the House of Commons as a Liberal. Defeated in the elections of 1859, he was appointed governor-general of British North America in 1861, becoming the first governor-general of Canada in 1867. The following year he resigned and returned to Ireland. His term of office was notable for his successful efforts to maintain peace between Great Britain and the United States during and after the American Civil War, and for the energetic part he played in bringing about the federation of British North America. Already the fourth viscount Monck of the Irish peerage, in 1866 he was raised to the peerage of the United Kingdom as the first baron Monck of Ballytrammon. In 1869 he was called to serve on the privy council. He served on the commission to carry out the Irish Land Acts from 1882 to 1884.