Mitchell-Thomson was the only son of William Lowson Mitchell-Thomson, 1st Baron Selsdon, and his first wife Madeleine McEacharn. His mother was an enthusiastic supporter of British club racing, particularly the Frazer Nash marque. She entered cars for Mitchell-Thomson in his early competitive outings and would stand in the pits, opening golf umbrellas bearing various symbols to relay information to him mid-race — an eccentric but effective communication system. He studied at Oxford University and married Phoebette Swithinbank in 1936, with whom he had one son, Malcolm, who became the 3rd Baron. Following their divorce he later remarried.
Mitchell-Thomson's motorsport career began in earnest at Brooklands in the early 1930s. Driving a Frazer Nash, he finished seventh in the 1933 BRDC 500 Miles Race and represented Oxford University in an inter-varsity race later that year. He returned to the 500-mile event in 1934 and 1935 without a classified finish but remained active across various British club events. In 1934 he participated in the Alpine Trial, a gruelling six-day, 1,970-mile endurance test from Nice to Munich, helping the Frazer Nash team to second place in their group. He also finished sixth in class and fifteenth overall in the Ulster Tourist Trophy that year, earning the Selsdon Bowl from the Frazer Nash Car Club for his all-round performance.
For the 1939 24 Hours of Le Mans, Mitchell-Thomson acquired a Lagonda V12 developed under W.O. Bentley's direction. The car was tuned to a theoretical top speed of 142 mph. Running conservatively to a pre-set average, and sharing with Lord William Waleran, the pair secured fourth place — an impressive result for a new and untried design. Shortly before the outbreak of World War II he set the fifth-fastest lap at the final Brooklands BARC meeting, averaging 128.08 mph, and travelled to Belgium for the Liège Grand Prix, which was cancelled as mobilisation orders came through.
After the war, Mitchell-Thomson purchased a Talbot-Lago T26C grand prix car in 1946. He shared it at the 1947 French Grand Prix with Yves Giraud-Cabantous but retired with engine failure. He sought a reserve entry for the 1948 British Grand Prix but did not convert it into a race start, and also entered the car for other drivers including Louis Chiron at various events.
In 1949 Mitchell-Thomson purchased a Ferrari 166 MM barchetta and entered it at Le Mans alongside the veteran Italian-American driver Luigi Chinetti. Acknowledging Chinetti's superior pace, Mitchell-Thomson took just a single driving stint of approximately one hour, starting at 4:26 a.m. when the car already held a three-lap advantage. The cautious, measured strategy was vindicated: the duo won outright, giving Ferrari its first of what would eventually become twelve overall victories at Le Mans. Mitchell-Thomson returned to Le Mans in 1950, this time partnered by Jean Lucas, but Lucas crashed from sixth place around 8 a.m. and the attempt ended there.
Beyond driving, Mitchell-Thomson became involved in the automotive industry as one of three directors of the HRG Engineering Company from 1947 until his death in 1963. He also owned a 1903 Mercedes 60, once considered the fastest production car in the world, which he entered in various heritage events including several editions of the London to Brighton Run and the 1936 Tilburstow Hill Climb. He sold the car in 1953; only four examples of the model are thought to survive.
Mitchell-Thomson made a brief cameo appearance in the Will Hay comedy film Ask a Policeman, in which a car chase leads to the Brooklands circuit and a fictional motor race.
Lord Selsdon's name is permanently linked to Ferrari's maiden Le Mans victory, one of the landmark moments in post-war motorsport. His role was primarily that of a wealthy privateer and enabler — providing the car, the entry, and the opportunity — while recognising Chinetti's talent and stepping aside accordingly. He served on the BARC Committee in 1955 and remained a respected figure in British motorsport circles until his death in 1963, when he was succeeded as Baron Selsdon by his son Malcolm.