[2][3][4][5] Lewis was born in London in 1920 and educated at University College School and St John's College, Oxford (MA 1945, DPhil 1950; James Mew Arabic Scholar, 1947).

At St John's College Lewis initially studied Classics.

With the outbreak of the Second World War, he served from 1940 to 1945 as a radar operator in the Royal Air Force.

He returned to Oxford in 1945 with his newly acquired interest in Turkish and on the advice of H. A. R. Gibb took a second BA degree in Arabic and Persian as groundwork for Ottoman Turkish, which he finished with first-class honours (not achieved in this double subject since Anthony Eden in 1922[6]) in just two years.

He spent six months in Turkey before pursuing his doctoral work on a medieval Arabic philosophical treatise at St John's College.