Amwell was the son of John Montague and Mary Ann Manderson.
He served in the First World War, achieving the rank of Lieutenant in the 1st Battalion of the Northamptonshire Regiment.
In 1923 Amwell was elected to the House of Commons as Member of Parliament (MP) for Islington West, a seat he held until 1931 and again from 1935 to 1947, and served under Ramsay MacDonald as Under-Secretary of State for Air from 1927 to 1931.
He was made a CBE in 1946 and in 1947 he was raised to the peerage as Baron Amwell, of Islington in the County of London.
Lord Amwell married Constance Mary, daughter of James Craig, in 1911.