Cahill became a professional tennis player in 1984 and won his first double title on the ATP World Tour the following year in Melbourne alongside Peter Carter . He was also in the final of the Queen's Club Championships with Mark Kratzmann . In total, he won 13 ATP double titles in his career, eight of them with Mark Kratzmann, including the Masters tournament in Cincinnati . Another twelve times he was in a double final.
In 1988 he won his first individual title in Gstaad , in the final he defeated Jakob Hlasek . He won his second and last individual title in 1991 in San Francisco . He reached his highest scores in the ATP world rankings in 1990 with 22nd place in singles and 10th place in doubles.
Cahill played six singles and four doubles games between 1988 and 1991 for the Australian Davis Cup team , where he was able to win all four doubles games. In the final against the United States in 1990, he did not play doubles as in the previous two rounds, but singles. He won his first game against Michael Chang , but had to give up injured against Andre Agassi when the score was 1-1 after sets; the USA finally won 3-2. At the Olympic Games in 1988 he competed in doubles for Australia, at the side of John Fitzgerald he reached the second round, in which they were defeated by New Zealanders Kelly Evernden and Bruce Derlin .
Cahill retired from professional sports in 1994 after suffering a knee injury. As a tennis coach, he led Lleyton Hewitt and Andre Agassi to the top of the world rankings. He was also the assistant coach of the Australian Davis Cup team from 2007 to 2008. In 2009 he turned down an offer from Roger Federer who wanted to sign him as a coach. He is currently training the Romanian top player Simona Halep .