He began his career at Ajax under manager Rinus Michels. In his first season at the club he did not play much. After the departure of left back Theo van Duivenbode in the summer of 1969 to Feyenoord, Krol became a regular player. When Ajax reached the UEFA European Cup in 1971, and won, Krol did not play because of a broken leg. Krol did play in the European Cup finals of 1972 and 1973. While others such as Johan Cruijff and Johan Neeskens left for new pastures, Krol, captain since the departure of Piet Keizer in October 1974, stayed at Ajax until June 1980.