He started cycling late, at the age of 18. Proost was active as a professional cyclist from 1958 to 1967. In 1960, 1961, 1963 and 1964 he took part in the Tour de France and won a stage in 1960. In 1961 he won a stage of the Giro d'Italia and in 1963 he finished second in the Belgian championships. 1964 followed his participation in the Vuelta a España , in which he reached the 22nd place.
After his career, he (who before his time as a professional driver had worked in a radio equipment factory) opened Café Waregem in Lier, Belgium. There he died after a long illness in Heilig Hartziekenhuis.