Right half Billy Dunlop, who also often played at centre half, was born in Annbank, South Ayrshire, and began his football career playing for his hometown club Annbank in 1892 before coming to England to sign for First Division club Sunderland in January 1893. He made his Football League debut on 28th January 1893 in a 4-2 home win against The Wednesday, and played four more matches at the end of the 1892-93 season once the League Championship was secured. From the following season onwards, Dunlop was a regular in the first team, contributing to their runners-up position in 1894 and a further League Championship triumph in the 1894-95 season, making 148 appearances over seven seasons at Newcastle Road and (in his final season) Roker Park, scoring 6 goals for The Wearsiders.
He also played on the winning side in a post-season friendly match against Scottish champions Heart of Midlothian, dubbed by some the “1895 World Championship”. He returned to Scotland in 1899 to join Glasgow Rangers and made his debut in a 5-1 win at Third Lanark that August. However Dunlop played only 6 Scottish First Division matches and one Scottish Cup tie in the 1899-1900 season, all victories, in each case covering for one of the regular half-backs, Neilly Gibson, Bobby Neil and Jacky Robertson, and four more Inter City League matches, all defeats, during the latter part of the season. He then moved on to Partick Thistle and played 3 matches in August 1900 for The Jags before returning to Annbank in October 1900, where he finished his career.