Mills was assistant at Fulwell Golf Club from 1948. He was in the RAF doing his National Service in 1950 and 1951 which limited his playing opportunities. He did, however, reach the semi-final of the Gor-Ray Assistants’ match-play tournament in 1950 and in 1951 won the Coombe Hill Assistants’ Tournament by 11 strokes. In late 1951, he moved to Wentworth as an assistant and was second to Bernard Hunt in the 1952 Coombe Hill Assistants’ Tournament. He lost in the final of the same tournament in 1954 at the 19th hole, but in 1955 he continued his success at Coombe Hill winning the tournament for a second time. Mills had moved to West Hill Golf Club in late 1954. Later the same year he won the Professional Short Course Championship in Torquay. He scored 202 for the 72 holes to win by two strokes and take the first prize of £100.
Mills twice qualified for the British Ryder Cup team, in 1957 and 1959. In 1957 selection for the team of 10 was based on a points system over a two-year period ending after the 1957 Open Championship. Winners of the 1957 Open Championship and the 1957 News of the World Match Play were guaranteed places with the remaining places selected from the points list. Following the 1957 Open, Mills was 10th in the points list and did not qualify at that stage. Only when Christy O’Connor Snr, who had already qualified for the team, beat Tom Haliburton in the final of the News of the World Match Play, did Mills became the last member of the team. In the 1957 match, Mills was not selected for the foursomes but played Jack Burke, Jr., the American captain, in the singles. Mills won 5 holes in a row from the 6th to the 10th to be 5 up, finished the morning round 5 ahead and eventually won the match 5&3. Selection for the 1959 team had changed, with seven members of the team chosen using a points system based on performances during the 1959 season, starting with the PGA Close Championship on 8 April and finishing with the Irish Hospitals Tournament on 12 July. Mills had a number of top-10 finishes and was fourth in the final list. Mills had a back injury while the team was in Washington and it was reported that he was to be replaced by John Panton. However, he recovered enough to remain in the team, although he was not chosen to play any matches.
Mills never won a major tournament although he was joint winner of the 36-hole Bowmaker Tournament at Sunningdale in 1958. He completed the front-9 of his second round in 28 (3-3-3-2-4-4-4-3-2) to equal the British tournament record. He was twice runner-up in the Daks Tournament at Wentworth. In 1956 he was joint-second, a distant 9 shots behind the winner, while in 1959 he was second on his own, three strokes behind Christy O’Connor Snr.