He played his entire career for Canterbury-Bankstown in the New South Wales Rugby League (NSWRL) competition.
In 1975, Cassilles played a handful of games for Canterbury and featured in the club's semi final defeat against Parramatta.
Cassilles played in the match as St George raced out to a 17-2 lead at halftime before going on to win 17-13 after a second half comeback by Canterbury.
Following the grand final defeat, Cassilles suffered a number of recurring injuries which limited his playing time for Canterbury.
[2][3] Cassilles later became a life member of the Canterbury club and was one of the players nominated for the Berries to Bulldogs 70 Year Team of Champions.