Casboult, 24, enjoyed a breakthrough season in 2014, breaking into the Blues' team in round four and playing 19 consecutive games and kicking 15 goals.
The 199cm forward also established himself as one of the strongest pack marks in the competition, finishing the season ranked ninth in the AFL and first at Carlton for contested marks.
Casboult came out of contract at the end of this season and attracted considerable interest from rival clubs, with the Western Bulldogs reportedly leading the chase.
Carlton football manager Andrew McKay said securing Casboult and Graham added to the "terrific mix of young talent" the Blues were assembling on their list.
"Levi has solidified a place in our forward line this year and has noticeably grown in confidence," McKay said.
"Nick has shown great potential and demonstrated his ability to be a ball-winner in the midfield.
"We believe both players will play key roles in our team going forward."
Casboult joined the Blues at the end of 2009 as pick No. 44 in the rookie draft. He took two and a half seasons to break into the Blues' senior team, but has now played 36 games.
Graham, 20, has proved an astute selection for the Blues at pick No. 54 in the 2012 national draft.
After playing two games in his debut season last year, the 2012 Morrish medallist played eight games this season, including six in a row from rounds 17 to 22, and impressed with his classy ball use and football smarts.
In first six games with the Blues, Graham played as the substitute, but the midfielder has been in Carlton's starting 21 in three of his past four games.
