His initial athletics breakthrough came at the World Junior Championships in 1992 as part of the British 4x400 metres relay team that finished fifth. Thomas attended Stamford School and West London Institute of Higher Education, where he studied Sports Science and Geography. While still at university, he took part in the 1994 Commonwealth Games 400 m where he reached the semi-finals, representing Wales. From there, he was selected to represent Britain in the relay at the 1995 European Cup and secured his first championship gold medal.