His breadth and depth of knowledge was so widely admired that for decades he was invited by other scholars to comment upon and improve a high proportion of all book manuscripts in the field of Greek history before they went to publication.
In his obituary in The Guardian newspaper (16 July 1994), it was stated that Prof. Lewis "has been, for the last two or three decades, the world's leading authority in the field of Greek epigraphy."
He was also an authority in several ancillary fields, "a profoundly learned scholar in Greek history" according to the Princeton Alumni Weekly memorial tribute.
Much of his work was concerned with inscriptions, but he insisted (in particular in an address to the epigraphic conference in University of Cambridge in 1967) that epigraphy should not be an activity for initiates only, but should be one means among others to the end of understanding the ancient world as fully as possible.
He also edited several Oxford publications in the classics, including The Decrees of the Greek States published posthumously in 1997.