Hilles was a fellow, and later Bodman Professor of English Literature, at Trumbull College, Yale University. He was a noted authority on the writings of Sir Joshua Reynolds and edited Reynolds letters which were published by Cambridge University Press in 1929. His book on The literary career of Sir Joshua Reynolds (1936) was an important source for Robert R. Wark's editing of the 1959 edition of Reynolds's Discourses."Preface" by Robert R. Wark in Joshua Reynolds, Discourses on art, 2nd edn, Yale University Press, New Haven, 1997. p. vi. Hilles' book was dedicated to his friend and mentor Chauncey Brewster Tinker. Later, Hilles was described as "the greatest living Reynolds collector".British Studies Monitor Vols. 4-6 (1973), p. 48.
