Under the direction of W. O. Bentley, Lagonda Ltd. redeveloped their V12 road car for the 1939 24 Hours of Le Mans. The 4½ litre engine was tuned for a theoretical top speed of and the car weighed just . Lagonda built two cars for the race; Mitchell-Thomson purchased one and entered with Lord William Waleran as co-driver. The team gave strict instructions not to exceed a pre-determined average speed based on the 1938 event, to ensure that the untried design went the distance. The cars did not challenge the likes of Bugatti and Delage, the former setting a new distance record, but impressed the British motoring press by securing third and fourth positions. Despite this promising performance, the outbreak of the Second World War prevented any further development of the model. Mitchell-Thomson piloted his Lagonda car to second place in the B.A.R.C. August Meeting at Brooklands, the last ever meeting at the circuit. He set the fifth-fastest lap of the meeting, averaging . He travelled to Belgium for the Liège Grand Prix, scheduled for 27 August 1939 and held at the site of Expo 1939 Liège. He set the third-fastest practice time, but the event was cancelled due to the mobilisation of troops and war began within days. In October 1939, he was reported as being on the Police Reserve.