Mildh was Finnish champion three times in the 400 m hurdles and once in the 110 m hurdles. At the 1954 European Championships in Bern he won an unexpected bronze medal in the 400 m hurdles in 51.5. The time which was a new Finnish record, would remain his personal best and was only broken as the record of his club, HIFK, in 2014. He won another bronze in the 4 × 400 metres relay as part of the Finnish team; Finland originally finished fourth, but moved up a spot when Great Britain was disqualified.