Mass sold a painting of the 1834 fire at the House of Commons, at that time attributed to J. M. W. Turner, to Paul Mellon, and sold a painting by Pietro da Cortona to the Metropolitan Museum of Art. He was also involved in the rediscovery and sale of many lost works, including Frederic Leighton's "Flaming June" (1895), and a painting of Saint Cecilia by John William Waterhouse that was found rolled up in France.