From 1945 Willem Kersters studied at the Royal Conservatory of his native town Antwerp, where he earned first prizes in solfège, harmony and piano. He then furthered his studies at the Royal Conservatory in Brussels under Jean Louël (counterpoint), Jean Absil and Marcel Quinet (fugue), Marcel Poot (composition) and René Defossez (orchestral conducting).

After receiving his diploma in music education, he worked for several years at secondary schools in Tienen, Leuven and Aarschot. From 1961 to 1968 he was a music programmer at the regional broadcast in Limburg of the BRT (Belgian Radio and Television). From 1962 he also held functions at the Royal Conservatory in Antwerp, first as a teacher of harmony, and later as the head composition teacher. In addition, he became the head of the theory department at the Maastricht Conservatory in 1967. Among his pupils may be cited Alain Craens, Koen Dejonghe, Jan De Maeyer, Luc Van Hove, Wim Henderickx and Marc Verhaegen. In 1989 he stopped teaching at these two conservatories.

As a composer, Willem Kersters received numerous awards both in Belgium and abroad, including the Second Count de Launoit Grand Prize in the International Queen Elisabeth Composition Competition in 1961 for the ballet Triomf van de Geest (Le Triomphe de l'Esprit, 1959), the City of Trieste Prize in 1963 for his Second Symphony (1963), and an award for his Second String Quartet (1964) in the International Composition Competition for String Quartets in Liège in 1965. In 1968 and 1972, he won two awards for his complete output, the Eugène Baie Prize from the Province of Antwerp and the SABAM Prize (from the Belgian asscociation for authors' rights). Kersters composed the obligatory piano concerto for the 1978 edition of the Queen Elisabeth Competition. At the end of 1990, he was elected a member of the Royal Academy for Sciences, Letters and Fine Arts of Belgium.