Kestergat started his career in journalism as an intern at "Le Phare" in 1946. On 12 September 1946, he married Jacqueline . In 1950, he started to work at "La Libre Belgique", where he would stay until his retirement in 1987. Initially, Kestergat reported on domestic Belgian matters, until he undertook his first foreign mission in 1957. The next year, he reported on the Belgian Congo for the first time, which would become his main subject for the rest of his career. In that context, he reported on the Shaba I and Shaba II wars in 1977 and 1978, as well as bilateral Belgo-Congolese (or Belgo-Zairean) relations under President Mobutu Sese Seko.