Cayless made his first grade debut for Parramatta in round 13 of the 1997 ARL season against the South Queensland Crushers which ended in a 52–10 victory at Parramatta Stadium. In the 1998 NRL season, Cayless made 19 appearances as Parramatta reached the preliminary final but were defeated 32-20 by rivals Canterbury-Bankstown in extra-time after leading the game 18–2 with less than 10 minutes to play. The following year, Cayless played in the club's heartbreaking 18–16 loss against eventual premiers Melbourne in the preliminary final after leading 16–0 at half time. In the 2000 NRL season, Parramatta again reached the preliminary final but were defeated by the Brisbane Broncos 16–10 at Stadium Australia. In the 2001 NRL season, Cayless captained the all conquering Parramatta side to the Minor Premiership where they set numerous records throughout the year. Cayless captained Parramatta in the 2001 NRL Grand Final against Newcastle where the club were heavy favorites to take out their first premiership since 1986 but suffered a shock 30–24 loss with Newcastle racing out to a 24–0 lead at half time. In the 2005 NRL season, Cayless captained Parramatta to their second minor premiership in five years and were favorites along with St. George to take out the title but suffered a shock 29–0 loss to North Queensland in the preliminary final at Telstra Stadium. In the 2009 NRL season, Cayless played nearly every game as Parramatta went on a remarkable run to reach the 2009 NRL Grand Final against Melbourne. Parramatta lost the final 23–16 at Telstra Stadium but Melbourne's premiership win was stripped the following year due to multiple and deliberate breaches of the salary cap. With Parramatta, he holds the record for the most games as captain of any team in the NRL. Cayless was never a prolific try-scorer; the most he ever scored in a season was 5. He also endured a scoring drought spanning over 80 games in his last several NRL seasons. In the final game of his career, Cayless finally brought the drought to an end, touching down in front of his home crowd. He also kicked a Field Goal in 2008 against the Newcastle Knights to send the match into extra time. Luke Burt then kicked the winning Field Goal. In 2014, Cayless was inducted into the Parramatta Eels hall of fame.
New Zealand
Cayless was selected for the New Zealand team to compete in the end of season 1999 Rugby League Tri-Nations tournament. In the final against Australia he played from the interchange bench in the Kiwis' 22–20 loss. Cayless was part of the Kiwis squad for the 2000 Rugby League World Cup, in which they reached the final. Cayless was selected to play for the New Zealand national team as a prop forward in the 2007 ANZAC Test loss against Australia. Cayless captained New Zealand to victory in the 2008 Rugby League World Cup. Cayless became one of the New Zealand national rugby league team' most-capped players, having appeared on 38 occasions by April 2009 when he retired from international rugby league, stating that he wanted to focus the rest of his football career on the Eels.
