Despite interest from bigger clubs, Thomas went on to make nearly 300 appearances for Swindon, including the 1969 League Cup final. By the time he left in 1973, he had made the most international appearances ever by a Swindon player, having played in thirty matches for Wales. He moved on to Derby County, when former Town manager Dave Mackay offered £100,000 for Thomas' signature. At Derby he was part of the team that won the First Division in 1975.