In both 1932 and 1933, Mitchell-Innes captained the Sedbergh School cricket team, and was invited to play at Lord's Cricket Ground for representative school sides, and he also made eight further County Championship appearances for Somerset. He failed to make a mark for the county in 1932, recording a batting average of 6.50, but in 1933 he achieved his first half-century in first-class cricket, scoring 57 against Warwickshire, before hitting his own wicket. In a review of their cricket side in July 1933, The Sedberghian says of Mitchell-Innes that "such cricketers seldom come this way", praising his consistency, fielding and captaincy, though it does note that his off drive often failed to score him boundaries, and that his bowling lacked accuracy at times. Mitchell-Innes also played fives and rugby for the school, and was president of the debating society. On completion of his studies at Sedbergh, he won an exhibition to study Law at Brasenose College, Oxford.