The 19-year-old has agreed a three-year deal, running until the summer of 2023.
Casey has been a regular for Carlos Corberan’s Under-23s throughout the campaign, making a total of 22 appearances in the Professional Development League and Premier League Cup.
His performances caught the eye of head coach Marcelo Bielsa who has included Casey in his matchday squad on a regular basis, with the defender making his senior debut from the bench in the 2-0 victory over Huddersfield Town last December.
He becomes the latest young player to sign a new deal with Leeds United, as the club continues to build for the future.