Whistler said that Keene was "the greatest English artist since Hogarth". He "possessed a gift of epigrammatic brevity hardly second to that of Leech or Gavarni" (Dictionary of National Biography).

He was permanently sketching, using whatever was to hand: '…for many years he made a practice of carrying an exciseman's ink bottle tied to his waistcoat button and a sketchbook in his pocket, so as to be ready to catch any passing incident, face or expression' (George Somes Layard, The Life and Letters of Charles Keene, London, 1892, p. 114).