Denny, Stirlingshire born right back Billy Forbes began his career with his hometown club, Denny Hibernians, and was a reserve for the Scottish junior international side. In August 1911 he moved south to join Southern League Plymouth Argyle, beginning a 13 year relationship with the club and making his debut that September at Bristol Rovers. After a slow start to his time at Home Park, he became a regular member of the first team during the 1914-15 campaign but the First World War interrupted his progress. During the conflict he worked at, and played for, the Devonport Dockyard. A versatile defender, he could play at right or left back, or as a half back, but is best remembered for his tremendous full back partnership with Moses Russell, alongside whom he played 181 times.
When football resumed after the War, Forbes was a fixture in the Argyle first team for the next five years, playing in their inaugural Football League fixture against Norwich City in August 1921, and being a member of the Pilgrims’ team that finished runners up in the Third Division (South) for three successive seasons between 1922 and 1924 narrowly missing out on promotion. He made his final appearance for the club in April 1924, but did accompany the team on the tour of South America that summer, and was still on the books at the beginning of the following season, having scored once in 250 games for The Pilgrims. He went on later that year to play for Fall River Marksmen in Massachusetts in the nascent American Soccer League before he retired from professional football shortly after.