Cahill was a professional tennis player in 1984 and won the following year in Melbourne at the side of Peter Carter his first doubles tournament on the ATP World Tour. In addition, he stood with Mark Kratzmann in the final of Queen 's Club Championships. In total he won during his career 13 ATP doubles titles, eight of them with Mark Kratzmann, including the Masters tournament in Cincinnati. A further twelve times he stood in a double finals. In 1988, he won his first singles title in Gstaad, in the final he defeated Jakob Hlasek. His second and last single title he won in 1991 in San Francisco. Its highest listing in the tennis world ranking he reached in 1990 with position 22 in singles and 10 in doubles position.
His best individual result at a Grand Slam tournament was reaching the semi-finals of the U.S. Open in 1988 when he defeated Boris Becker in the second round and was only subject to the eventual tournament champion Mats Wilander. In the doubles competition, he stood in 1989 at the finals of the Australian Open, on the side of Mark Kratzmann he was defeated by the Americans Rick Leach and Jim Pugh.
Cahill played between 1988-1991 six singles and four doubles matches for the Australian Davis Cup team, where he won all four doubles matches. In the final against the United States in 1990, he did not present himself as in the two previous rounds in a double, but in singles. He won his first match against Michael Chang, but had to give up in the match against Andre Agassi at the registry of 1-1 injured in the sets, the USA eventually won 3-2. In the Summer Olympics 1988, he stepped up to double for Australia, at the side of John Fitzgerald, he reached the second round, where they were the New Zealanders Kelly Evernden and Bruce Derlin inferior.
Cahill joined in 1994 after a knee injury from professional sports back. As a tennis coach, he led Lleyton Hewitt and Andre Agassi at the top of the tennis world rankings. He was also from 2007 to 2008 assistant coach of the Australian Davis Cup team. In 2009 he declined the offer of Roger Federer to be the coach.