 De Keersmaeker began his professional career as manager of the family brewery De Keersmaeker N.V. in Kobbegem.From 1959 until 1985 he was mayor of Kobbegem/Asse, and from 1968 to 1992, he was a member of the Belgian Parliament.From 1974 to 1981 he was a member of the European Parliament, and from 1981 to 1992 Member of the Federal Belgian Government as Secretary of State for European Affairs and Agriculture.