Mitchell-Innes made his debut for Somerset while he was still a schoolboy at Sedbergh School in 1931. He subsequently went to Oxford University and appeared in the annual match against Cambridge in each of his four years. His total of 3,319 first-class runs is a record for the Oxford University team, and he is regarded as one of the best university cricketers ever. After completing each year at Oxford, he returned to play for Somerset. He played his best years of cricket while at university, passing 1,000 runs in the season during three of his four years there. After graduating from university, he joined the Sudan Political Service and missed the 1938 cricket season entirely. He was only available for Somerset during periods of leave thereafter, often playing for around four to six weeks. In 1948, he was one of three players to captain Somerset when the county struggled to appoint anyone on a permanent basis. He played his last first-class matches in 1949.