Mathias first played tag rugby for Llangwm RFC, before stepping up to Narberth RFC, where he played alongside future Scarlets teammate Ryan Conbeer.[2] Mathias joined the Scarlets academy, and was also registered to local clubs Carmarthen Quins RFC and Llandovery RFC.[2]
Scarlets
Mathias was named in the Scarlets first-team squad for the 2020–21 Pro14 season.[3] Originally slated to make a possible debut off the bench along with fellow academy product Iestyn Rees in the EPCR Challenge Cup match against Toulon on 18 December 2020, Rees was forced to wait, as the match was called off due to COVID-19 cases.[4][5] He finally made his Scarlets debut in Round 11 of the 2020–21 Pro14 against Cardiff Blues.[6] On 29 May 2021, Mathias signed his first professional contract with the Scarlets.[7]
Mathias scored his first try for the Scarlets on 27 January 2023, in the 37–28 win over the Bulls.[9] Mathias signed a contract extension with the Scarlets on 20 April 2023, along with fellow prop Sam Wainwright.[10]
International career
Wales U18 and U20
While attending Sir Thomas Picton School, Mathias was selected for Wales U18 on their tour to South Africa in 2017, starting against England and scoring a try in the win.[11] The following year, Mathias was first selected for Wales U20, and made 11 appearances for the side over two seasons. Mathias was part of the team what beat New Zealand U20 in the 2019 World Rugby Under 20 Championship.