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.