He was the youngest son of Charles de Berlaymont and Adriana de Ligne Barbançon, and brother of Gilles de Berlaymont and Claude de Berlaymont. In 1576 Florent shortly joined the Union of Brussels, but soon became loyal to the King of Spain again. In 1579 he succeeded his brother Gilles, who was killed at the Siege of Maastricht (1579), as stadtholder of Namur and Artois.