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]