Casboult, a tall and solidly built player, spent much of his formative years as a ruckman and key position player. Casboult began to play TAC Cup football with the Dandenong Stingrays in 2007. He played a total of 44 games with the Stingrays; in his first two seasons, he scored only three goals, but in 2009 he was moved into a key forward role and scored 41 goals in 20 matches.

Casboult was recruited by the Carlton Football Club with its third round selection in the 2010 AFL Rookie Draft (at selection 44 overall). His first month at the club was eventful. Before the 2009 Christmas break, Casboult attended the team’s controversial booze cruise (the fallout from which included the suspensions of Andrew Walker, Eddie Betts and Ryan Houlihan, plus a “please explain” to the club from the AFL); it was widely reported in the media that, in an initiation game, Casboult was handcuffed to an unnamed senior player and forced to match him drink-for-drink until barely conscious. A few days later, on 24 December, Casboult was fortunate to walk away uninjured from in a head-on car accident: his car was struck by the car of Samir Azemi, which had jumped the median strip on the Princes Highway in Dandenong. Azemi was killed in the accident.

Casboult won a regular place in the senior team in 2014, playing as a forward and sometimes as a back-up ruckman. Over the next few seasons, he gained attention as one of the strongest contested marks in the competition, and he was a Mark of the Year finalist in 2014. Through the early part of his career, he was noted for very poor goalkicking accuracy from set shots, returning less than 50% each season; but after working with former Collingwood and North Melbourne full forward Saverio Rocca in 2015 and again in 2017, became one of the team’s most reliable set shots.