Thomas was born in Mochdre, Conwy, Wales, and was among a group of notable footballers to come out of North West Wales during the 1970s and 1980s. Local factory Quinton Hazell bought him a pair of boots when he was at school in the mid 1960s, and at the age of 13 Thomas was a success as left wing on the factory's Conwy League men's team. By 15, he and friend Joey Jones were taken on by Wrexham. Though the first two years were spent cleaning boots, the changing rooms and the whole stadium, Thomas made his first-team debut in the 1971–72 season, when still only 17. Under manager John Neal he helped Wrexham establish their giant-killing reputation by reaching the quarter finals of the FA Cup in 1974 and the quarter finals of the European Cup Winners' Cup in 1976, when they lost to the eventual winners, Anderlecht of Belgium. After helping Wrexham storm to the Third Division title in 1977–78, Thomas crossed the border to England and joined Manchester United. He played 110 games and scored 15 goals for the Red Devils, collecting an FA Cup runners-up medal in 1979.