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.
Florent inherited the title of Count of Berlaymont from his father in 1578.
When his other brother Claude was killed in 1587, Florent succeeded him as stadtholder of Guelders-Zutphen.
In 1604, Florent also became stadtholder of Luxembourg, after the death of Peter Ernst I von Mansfeld-Vorderort.