In the early 1930s he worked as the chief assistant to Arthur Rowledge at Rolls-Royce on the R engine.
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.
For around a year, from 1938 to 1939, he worked as an independent consultant under the name 'Templewood Engineering', an owned subsidiary of High Duty Alloys Ltd. Wallace C. Devereux set up this offshoot company in order to market their Hiduminium range of high-performance aluminium alloys to the motor-racing industry.
In 1939 he became the Chief Engineer of Armstrong Siddeley, based north of Coventry (now Rolls-Royce Ansty).
Tresilian designed an oversquare twin-cam 2.5-litre four-cylinder for the BRM P25 Formula One car.