1936: two-year-old season

Mid-day Sun did little in his two-year-old season to suggest that he was a potential Classic winner. He lost his first six races, his best effort coming when he finished sixth to Perifox in the Richmond Stakes at Goodwood. At Newmarket on October 13 Mid-day Sun recorded his first win when carrying a light weight in a one-mile Nursery (a handicap race for two-year-olds). He then finished second in a similar event at the same course two weeks later, failing to give seventeen pounds to the winner.^[5] At the end of the season he was rated on 99 pounds, more than thirty pounds below the top-rated colt Foray.^[4]

1937: three-year-old season

On his three-year-old debut, Mid-day Sun won the Free Handicap at Newmarket in April, receiving six pounds from the filly Exhibitionnist. The form of the race did not look particularly strong, but was boosted later in the year when the runner-up won both the 1000 Guineas and the Oaks. On his next start, Mid-day Sun proved himself capable of competing with the best colts by finishing third of the eighteen runners behind Le Ksar and Goya in the 2000 Guineas. He then prepared for the Derby by winning a slowly run race for the Lingfield Derby Trial. In the build-up to the Classic he was reported to be "in demand" in the betting market but was not among the favourites.^[6]

At Epsom, Mid-day Sun was ridden by the Irish jockey Michael Beary and started at odds of 100/7 (just over 14/1) in a field of twenty-one. The race was run in bright sunshine in front of the customary immense crowd including the King and Queen.^[7] Mid-day Sun was held up in the early stages as Renardo and then Fairford led the field. Le Grand Duc moved into the lead from Goya in the straight, heading a closely packed group of seven horses, with Beary moving his horse into contention. Mid-day Sun produced a “dazzling run”^[8] to move through the middle of the group, and pass Le Grand Duc inside the final furlong. Once in front, he held off the late challenge of Sandsprite to win by one and a half lengths. Mrs Miller called the result "the moment of my life" before explaining that she would prefer to celebrate with "a cup of tea" followed by a quiet dinner with her husband and mother.^[9] Beary felt that the colt had been "running lazily" in the early stages, but said that he had "responded gallantly" at the finish.^[2] The quality of the race was, however, questioned by some including the Sunday Times who pointed out that the proximity of the 100/1 outsider Sandsprite made the form look suspect.^[10]

Mid-day Sun was then sent to Royal Ascot where he was matched against leading older horses including the American-bred Flares in the Hardwicke Stakes. He took the lead early in the straight and pulled clear to win decisively, returning to "a big reception".^[11] According to Butters, the race "took nothing out of him",^[12] and he was then aimed at the St Leger at Doncaster. He prepared for the Classic by winning the Manton Stakes at Salisbury on August 23. In the St Leger Mid-day Sun started favourite but Beary struggled to obtain a clear run in the straight^[13] and although the colt finished strongly he finished third, beaten just over a length by Chulmleigh with Fair Copy second.^[14]

1938: four-year-old season

Mid-day Sun was kept in training as a four-year-old and aimed at the Ascot Gold Cup.^[15] His temperament became a problem however and it proved impossible to train him effectively. He was retired to stud without racing in 1938.^[4]