Trevelyan was first a Liberal and later a Labour MP. His eventual political achievements were uneven. As a member of the landed gentry serving in the Labour Party, he was considered by some to be a walking anachronism. Despite this, his own privileges and gentlemanly pursuits always remained intact. Trevelyan was elected Liberal Member of Parliament (MP) for Elland, Yorkshire, in a by-election in 1899. He served under H. H. Asquith as Parliamentary Secretary to the Board of Education between 1908 and 1914, when, as an opponent of British entry into the First World War, he resigned from the government. In 1914, also, he founded the Union of Democratic Control an all-party organisation rallying opposition to the war. In the 1918 general election he lost his Elland seat, running as an Independent Labour candidate.