Son of Maurits Lodewijk II van Nassau-LaLecq (1670-1740) and Elisabeth Wilhelmina van Nassau-Odijk (1671-1729). He married firstly (1736) Cornelia d'Hangest d'Yvoy (1714-1744) and secondly (1749) to Maria Anna Testas (1715-1795). Floris had at least 5 children.
Floris became a Lieutenant of the Army in 1732 and chief of the Garrison of Doornik in 1738. He left the army in 1740. In 1755 he was elected Grand Bailiff of Utrecht, a position which he kept until ca. 1779.
When Boswell arrived in Holland in 1763, he brought with him an introduction to Count Nassau from David Dalrymple, who had studied in Utrecht some years earlier. In a letter to Temple (dated September 23, 1763) Boswell wrote that "Count Nassau is very polite, and tells me that by and by we shall have fine parties". While he became more intimate with other branches of the illustrious Nassau family, he only saw Floris, then Grand Bailiff of Utrecht, occasionally - on February 12, 1764, he "waited on Grand Bailiff after prayers and talked long with him and well".