[9] Thomas represented England U20 during the 2011 Six Nations Under 20s Championship and started in their decisive last fixture which saw them defeat Ireland to complete a grand slam.

[14] In January 2013 Thomas received his first call-up to the senior England squad as an injury replacement for Alex Corbisiero during the 2013 Six Nations but did not feature in the tournament.

[18] Thomas played in victories over Scotland, Ireland and Wales during the 2014 Six Nations Championship as England completed their first Triple Crown for over a decade.

[23] He qualifies for Wales through his Swansea-born father and is eligible under World Rugby regulations because he played for England more than three years ago.

[24] Thomas made his Wales debut on 5 August 2023, coming off the bench in the first test of their 2023 Rugby World Cup warm-up matches, in a win against former team England.