Cox had previously played basketball for Oklahoma State University in the Big 12 Conference and, standing at 211 cm (6 ft 11 in),[4] is the tallest player to ever be measured at an AFL Draft Combine.

[11] He initially joined the Oklahoma State Cowgirls' "scout team" to simulate Brittney Griner, former Baylor University basketball player.

[11] Cox was invited to attend the 2014 US International Combine for potential Australian rules footballers, in Los Angeles in April 2014 despite knowing very little at all about the sport.

[14] Cox played for the Collingwood reserves team in the Victorian Football League (VFL) throughout the 2015 season while learning the game and improving his skills.

Cox played well in the opening two games of the 2016 VFL season[19] including a dominant performance up forward against Port Melbourne and as such earned a long-awaited call up to the AFL.

[24] Cox continued his good form in the 2018 season, kicking a career-high five goals in the Queen's Birthday match at the MCG and was awarded the Neale Daniher trophy as best on ground.

After the 2019 injury, Cox said he had to spend two weeks in bed in a darkened room for 45 minutes of each hour, calling it "probably the darkest moment of my life.

Mason has expressed a keen interest in promoting the game at the grassroots level in the US, visiting the US to support his brothers at the USAFL National Championships.

[36][37] Cox was seeking permanent resident status on a path towards Australian citizenship to fulfill his desire to live the rest of his life in Australia, committing to a multi-year contract at Collingwood.