William Thomas "Billy" Mainwaring (born Gwilym Thomas Mainwaring; 24 January 1941 – 3 April 2019) was a Welsh rugby union player. He played as a second row. He played for Wales in 1967–68. He appeared for his country six times. He played for amateur teams Taibach, Aberavon, Bridgend and the Barbarians. He was born in Port Talbot. He worked as a steelworker.