By 1728 he had left Preston, and the following year he was working in London for the Flemish topographical and sporting painter Peter Tillemans. There he specialized in landscape painting and copying various works in Tillemans studio after Marco Ricci, Giovanni Paolo Panini and Jan van Bloemen. Devis earliest known commission, Hoghton Towers from Duxon Hill, Lancashire (1735; priv. col., see 1983 exh. cat., no. 3), painted for Sir Henry Hoghton during a trip to Preston in 1734-5, shows Tillemans influence in its attention to detail and the use of thin, transparent paint. Thomas Lister with his Family (c. 1738; Chicago, IL, A. Inst.) demonstrates a similar interest in landscape, featuring the family group in Gisburn Park, Lancs. Devis had returned to London by 1742 and established himself as a painter of conversation pieces, with a studio in Great Queen Street. Roger Hesketh with his Family is typical of his work at this time; it shows how Devis transformed the intimacy of a Dutch 17th-century genre scene into an elegant interior with the group of sitters connected by formal, schematic gestures. Roger Hesketh stands apart, in a tastefully contrived pose, his legs crossed and right arm thrust inside his waistcoat. His son, Fleetwood, stands with his hand resting on a dog next to his wife, who is seated with an infant on her lap. The adjacent telescope, globe and marine paintings are intended to advertise Hesketh interest in astronomy and travel.