The club was formed in 1945, and joined the Welsh League. In their first season, they finished as runners-up, and joined the Southern League. The club were immensely successful in their first few seasons, winning the championship in 1947–48, 1949–50, 1950–51, 1951–52 and 1953–54. In the 1947–48 championship-winning season, Merthyr only dropped one point at home and only lost four games all season. However, despite their success, the club failed to be elected to the English Football League. The 1950–51 season ended with Merthyr winning the Southern League, the Welsh Cup, the Southern League Cup and the Welsh Challenge Cup. After bouncing between the various Southern League divisions during the 1960s, 1970s and 1980s, the club finally won their sixth championship in 1988–89, making them joint Southern League title record-holders together with Southampton. This time, winning the championship meant promotion to the Football Conference, the fifth level of English football. The club finished ninth in their first two seasons, and then fourth. However, they then went into decline, and were relegated back to the Southern League in 1995, where they remained until reforming in 2010. They should have been relegated a season earlier, but were reprieved when Northern Premier League champions Marine failed to meet the stadium criteria for the league. The club's best FA Cup performance was reaching the second round, which they managed in 1946–47, 1954–55, 1973–74, 1979–80 and 1990–91. The only time the club managed to defeat a Football League club in the FA Cup was in the first round of the 1946–47 cup, when they beat Bristol Rovers 3–1. The club were considerably more successful in the Welsh Cup, which they won on three occasions: 1949, 1951 and 1987. In addition, the club finished as runners-up in 1947 and 1952. After winning the final in 1987, the club were allowed to enter the European Cup Winners' Cup, at a time when English clubs were banned from European competition. In the first round, they were drawn against the Italian club Atalanta. The club managed a win in the first leg at home, beating the Italians 2–1. However, the return leg was lost 2–0, and the club were out. Historically, the club's biggest rivals are Gloucester City. The two clubs played over 120 times in their history, making it one of the most played Anglo-Welsh derbies in football. In the late 1990s and early 2000s Newport County were local rivals but the teams have not met in the same league for many years.
