Born in Hackney, Benn was the second son of Sir John Benn, MP, and his father-in-law Daniel Holmes was also an MP. He was educated at the Lycée Condorcet in Paris and at University College, London. In 1906 he was elected as a Liberal MP for the St George's division of Tower Hamlets in east London, a seat he held until 1918. He served with the Royal Naval Air Service during World War I, seeing service at Gallipoli. He was a Liberal Whip 1910–1915. In 1918 he was elected for Leith in Scotland, a seat he held until March 1927, when he resigned from the Liberal Party and from Parliament. In 1920 he married Margaret Holmes.
In 1928 Benn was re-elected as a Labour member for Aberdeen North. He was Secretary of State for India in Ramsay MacDonald's second government (1929–1931). He refused to follow MacDonald into the National Government coalition with the Conservatives, and at the 1931 election he lost his seat.
His grandson, the Labour MP for Leeds Central Hilary Benn, is currently the Secretary of State for Environment, Food and Rural Affairs.