As a player, he spent most of his career in Dutch football, winning five Eredivisie championships with Ajax Amsterdam and two with PSV Eindhoven.

A left-footed midfielder with a powerful shot and great stamina, Lerby was a player who was always committed and a leader of the team.

He once played two competitive matches in two countries on a single day on 13 November 1985: first in the afternoon a World Cup qualifier with Denmark against the Republic of Ireland in Dublin, being substituted having helped to secure a solid lead, and later in the evening a DFB-Pokal match with Bayern Munich against VfL Bochum, entering as a half-time replacement.

He got his senior debut with Fremad Amager, playing 13 games and scoring three goals in the top-flight 1975 1st Division season.

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.

While at Ajax, he made his senior Danish national team debut against the Republic of Ireland in May 1978, scoring one goal in the 3–3 draw.