[1][3] Until he was 16 years old, Mathiang played Australian rules football, rugby, track, and soccer for Emmanuel College in the Melbourne suburb of Altona North.

In his senior season, he averaged 14 points, 10 rebounds, and 4 blocks per game as he helped the IMG Academy get a 28–2 record.

[7] Under the terms of the deal, he split the 2017–18 season with the Hornets and their NBA G League affiliate, the Greensboro Swarm.

[10] On 11 July 2019, Mathiang signed with Bahçeşehir Koleji of the Turkish Basketbol Süper Ligi (BSL).

[16] After a four-game stint with the Casey Cavaliers during the 2022 NBL1 South season,[17] Mathiang signed with the Illawarra Hawks of the National Basketball League on 29 July 2022.

[19][20] On 29 January 2023, Mathiang signed with Hapoel Eilat of the Israeli Basketball Premier League.

[22][23] On March 11, 2024, Mathiang signed with Maccabi Ironi Ramat Gan of the Israeli Basketball Premier League.

[25] Mathiang made his debut for the Australian national team in a 2019 FIBA World Cup qualifying match against Kazakhstan.

[26] In August 2023, Mathiang was named in the South Sudan national team for the 2023 FIBA World Cup.