Billy Moffatt was born in Bellshill, Lanarkshire, in 1897. He played junior football for Bellshill Athletic before joining Bo’ness of Division Two. He captained the team to the quarter-finals of the 1922-23 Scottish Cup, in which they performed well in a 4–2 defeat to Division One club Motherwell despite being handicapped by an injury to Billy.
In 1925, he signed for English Second Division club Portsmouth.club He soon established himself in the team, and was ever-present in the 1926–27 season in which they were promoted to the First Division, He took his appearance total to 138 before being released in 1930 to join Brighton & Hove Albion then of the Third Division South.
He was already 32 years old, and Albion used him as a standby player to cover at right back at and wing half. In his first season, he contributed to a run of 16 league matches unbeaten that remained a club record until 2015. After two seasons and 23 appearances, he left the Football League for non-league football.