As a junior, Casson competed for the Western Australian Under 17 and 19 teams as well as their Second XI team, however it wasn't until the 2002–03 season when he won his first full state contract. 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. With seven of Western Australia's players away on international duties, Casson made his first-class debut on 8 December 2003 against Tasmania at the WACA Ground. He had a quiet debut with the ball, taking 0/44 in Tasmania's first innings, but had a solid showing with the bat, scoring 35 runs in Western Australia's second innings. He had an excellent second first-class match as he tore through South Australia's batting line-up in the first innings to finish with 6/64 and went on to take 10 wickets for the match and was named Man of the Match for the first time in his senior career. 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. The 2002–03 season also saw Casson make his List A debut on 10 January 2003 against Victoria at the Melbourne Cricket Ground. He failed to take a wicket on debut, and in the three other matches he played in the ING Cup season.