Pronk came through the Ajax youth system and made his senior debut for the club on 4 September 1960 against PSV. He played in 337 official matches for them during the 1960s, winning 4 league titles and 3 KNVB Cups. He was on their losing side in the 1969 European Cup Final and was released by Ajax before the club won the Champions's Cup for three successive years in the beginning of the 1970s.