Thomas attended King Edward VII School in Houghton, Johannesburg, South Africa alongside Joe van Niekerk and Bryan Habana.
The 2004–05 season saw Thomas establish himself as a regular in the Dragons squad, making 27 appearances and scoring two tries, as the team finished in 4th place in the Celtic League.
Thomas made a further 30 appearances in 2005–06, but he was unable to prevent the team from slipping to 8th place in the Celtic League, forcing them to have to play a play-off match against Rugby Parma to determine qualification for the 2006-07 Heineken Cup.
Thomas had missed most of the season due to a back injury but was available for the Challenge Cup semi-final and the final three games of the Celtic League.
In 2006, Thomas made his debut for the Wales national rugby union team, coming on as a replacement for Adam Jones in the 57th minute of a match against Argentina on 17 June 2006.