James Purdon Lewes Thomas was the son of John Lewes Thomas, JP, of Cae-glas, Llandeilo, Carmarthenshire, and Anne Louisa, daughter of Commander George Purdon RN of Tinerana House, County Clare and Anne Caulfield.

He was educated at Rugby and Oriel College, Oxford, where he was awarded an aegrotat degree in French in 1926 (indicating that he was unable to sit the final examinations due to ill-health).

Thomas volunteered for military service at the outbreak of the Second World War in 1939, but was rejected due to a permanent knee injury.

After the 1945 general election Thomas was the opposition spokesman on naval affairs and deputy chairman of the Conservative Party, where he was responsible for recommending parliamentary candidates.

[1] When Churchill returned as Prime Minister following the 1951 general election, Thomas was sworn of the Privy Council and appointed First Lord of the Admiralty.