In the 1920s he worked at Rolls-Royce on aero-engines. In the early 1930s he worked as the chief assistant to Arthur Rowledge at Rolls-Royce on the R engine.
Lagonda
In 1936 Tresilian was invited by W.O. Bentley to follow him from Rolls Royce to Lagonda to become chief designer. The V12 Lagonda Tresillian designed followed the general principles he had set out at Rolls Royce for a new but unbuilt version of the Phantom III. Despite a lack of funding and development the V12 Lagonda was a great success.