While at Oxford, Clark was greatly impressed by the lectures of Roger Fry, the influential art critic who staged the first Post-Impressionism exhibitions in Britain. Under Fry's influence he developed an understanding of modern French painting, particularly the work of Cézanne. Clark attracted the attention of Charles F. Bell (1871–1966), Keeper of the Fine Art Department of the Ashmolean Museum. Bell became a mentor to him and suggested that for his B Litt thesis Clark should write about the Gothic revival in architecture. At that time it was a deeply unfashionable subject; no serious study had been published since the nineteenth century. Although Clark's main area of study was the Renaissance, his admiration for Ruskin, the most prominent defender of the neo-Gothic style, drew him to the topic. He did not complete the thesis, but later turned his researches into his first full-length book, "The Gothic Revival" (1928). In 1925, Bell introduced Clark to Bernard Berenson, an influential scholar of the Italian Renaissance and consultant to major museums and collectors. Berenson was working on a revision of his book "Drawings of the Florentine Painters", and invited Clark to help. The project took two years, overlapping with Clark's studies at Oxford.