Montagu was the second son of Samuel Montagu, 1st Baron Swaythling. He attended Clifton College and from 1898 to 1902 Trinity College, Cambridge. In 1906 he entered politics, being elected MP for Chesterton as a Liberal, and represented that constituency until 1922. He was successively Secretary to the Chancellor of the Exchequer from 1906 to 1908, Secretary to the Prime Minister from 1908 to 1910, Parliamentary Under-Secretary for State for India from 1910 to 1914, Financial Secretary to the Treasury 1914-16, Minister of Munitions 1916 and Secretary of State for India 1917-22. He died in 1924.
General Correspondence 1905-22, correspondence regarding Ireland 1914-21, correspondence and papers relating to India 1911-21, Indian tour diaries 1911-18, correspondence relating to the Turkish peace settlement 1918-21, Cambridge University Liberal Club minute books and officers' correspondence 1886-97.