While at Oxford, Mitchell-Innes had become good friends with Jake Seamer, who had also attended Brasenose College, and the pair were teammates for both Oxford and Somerset. Seamer had joined the Sudan Political Service (SPS) on his graduation in 1936, and while at Oxford, Mitchell-Innes also secured a role with the organisation that he would take up after his graduation. His duties with the SPS prevented him from playing for Somerset at all during 1938, but he made seven early-season appearances the following year, though two were for the Marylebone Cricket Club and the Free Foresters. It was for the latter of these that Mitchell-Innes made his only century of the season; playing against Oxford University, the Free Foresters required nearly 400 runs in the fourth innings to win. Mitchell-Innes opened the innings, and after the lunch interval on the final day he played attacking cricket, bringing up his century in 95 minutes. He was eventually dismissed for 143 runs, bowled as he tried to heave an on drive. The Free Foresters came within nine runs of their target, but were bowled out before they could reach it. In total, he scored 347 runs at 31.54 that year; the last season of first-class county cricket in England before it was interrupted by the Second World War.