[citation needed] Llewellin was commissioned into the Royal Garrison Artillery in 1914 and reached the rank of major during the First World War, winning the Military Cross in 1917.

He was appointed an Officer of the Order of the British Empire (OBE) in 1926,[2] promoted to a Commander (CBE) in 1939,[3] and then was made a Knight Grand Cross (GBE) in 1953.

He held a number of ministerial posts in the Coalition government, eventually serving as President of the Board of Trade for two weeks in 1942.

[citation needed] He subsequently became Minister of Aircraft Production until replaced by Sir Stafford Cripps in November 1942.

[5] Later, Llewellin served on the Combined Policy Committee set up by the British and United States governments under the Quebec Agreement of 1943 to oversee the construction of the atomic bomb.