[4][5] He was a vital part of the Australian Under 19 team that won the 2002 Under 19 Cricket World Cup in New Zealand,[6] taking 12 wickets at 15.08 in 6 matches.
[10] Prior to making his first-class debut, Casson also played for a Western Australian XI that took on the travelling English cricket team, taking the wicket of James Foster and finishing with figures of 1/32.
[11] With seven of Western Australia's players away on international duties,[12] Casson made his first-class debut on 8 December 2003 against Tasmania at the WACA Ground.
[14] Casson played the remaining games of the Pura Cup for Western Australia, however he faded with the ball towards the end of the season, finishing with 17 wickets at 39.35.
[28] Although Western Australia tried to keep him, Casson felt that bowling on the more spin-friendly pitches of the Sydney Cricket Ground would help his career.
[29] While there were doubts over whether Casson would be able to hold down a permanent place in the New South Wales team with four other spinners in the squad,[30] he managed to play seven matches in his first season with the Blues.
Despite this, he had a very poor first season, taking only 7 wickets at an average of 72.00,[31] however his form was not helped by a shoulder injury that eventually required reconstructive surgery.
[33][34] He hit a career top score of 99 off 180 balls against South Australia at the Sydney Cricket Ground and he played a vital role in the Blues' 2007-08 Pura Cup final win.
[38] Despite performing well in first-class cricket, Casson struggled to make the New South Wales one-day side with Nathan Hauritz being the first choice List A spinner for the Blues.