 His plays drew on a wide range of material and covered a range of subject matter including Welsh mythology and history, as well as the Bible, although he also wrote plays set in contemporary Wales.Lewis wrote the libretto for Arwel Hughes's opera ' (Love’s the doctor), based on Molière's ' (first performance 1960 by Welsh National Opera).He published two novels, "Monica" (1930) and ' (The daughter of Gwern Hywel) (1964) and two collections of poems as well as numerous articles and essays in various newspapers, magazines and journals.