Lerby was born in
Copenhagen, the son of former
Danish under-21 football international Kaj Lerby. Lerby played his youth football with local clubs
B 1903,
Taastrup IK and
Fremad Amager. He got his senior debut with Fremad Amager, playing 13 games and scoring three goals in the top-flight
1975 1st Division season. He and Fremad Amager teammate
Frank Arnesen moved abroad to become professional footballers in November 1975. They switched to the Dutch club
Ajax Amsterdam, when Lerby was only 17 years old. Lerby's first home game was a 4–1 against
Go Ahead Eagles on 11 April 1976. He won three Dutch
Eredivisie league titles in his first six years at Ajax, in the 1976–77, 1978–79, and 1979–80 seasons. In the summer of 1981 he was named
team captain, succeeding newly departed Frank Arnesen. In the two seasons Lerby was captain, Ajax won a further two Dutch championships with high goal differences. He moved to Germany in 1983, replacing midfielder
Paul Breitner at
Bayern Munich. He remained there for three years, winning two German
Bundesliga championships and two
DFB-Pokal cup trophies. His career continued for French team
AS Monaco from 1986 to 1987. Lerby moved back to the Netherlands in 1987 and was reunited with Frank Arnesen at Ajax' rivals PSV Eindhoven. He was a part of the 1988 European Cup winning PSV squad, also including fellow Danes
Jan Heintze and
Ivan Nielsen, though Frank Arnesen missed the final against
S.L. Benfica because of injury. Lerby ended his active career in 1990. ==International career==