After an undergraduate degree in Classics at Princeton University, Clay wrote a masters thesis at Oxford University on the coinage of Septimius Severus. He later collaborated with Andreas and Elisabeth Alfoldi in both volumes of their work (Berlin, 1976 and 1990) on Roman contorniates (an enigmatic class of late Roman medallions).

In connection with his interest in the chronology and historical interpretation of the coinage of the Roman Empire, he has presented papers at a number of international numismatic congresses and symposia. Articles by him have also been published in scholarly journals on such subjects as the purpose of the issue of Roman medallions, the coinages of Nero and Macrinus, and the supply of bronze coins to Britain under the Roman Empire.