[1] As his father Claes was the fourth surviving son of Wolfert I, one might expect that Floris would not get a rich inheritance.

However, in the end Floris did leave a very rich estate, probably due to his career in the service of the counts of Holland.

During the Hook and Cod wars, Knight Claes I van Borselen and his sons pledged loyalty to Margaret II, Countess of Hainaut in January 1351.

[2] At the same time Claes and his sons Wolfert and Hugeman promised to submit to a verdict that Margaret II, Countess of Hainaut would give about the killing of Arnoud van Haamstede.

This might have been due to them being too young, when Arnoud van Haamstede was killed about four years earlier, or that they were simply not involved.

[11] What is likely, is that after our Floris appeared alongside the other two, the clerks stopped using the label 'the younger' for the son of Rase, because it became confusing.

The regular or great seal, which was indeed bigger, was attached to open letters, or charters meant for a larger audience.

On 12 February 1353 Floris van Borselen son of Claes was entrusted with keeping this great seal.

[23] On 10 January 1354 Floris van Borselen bought 1600 gemeten of land, which had reverted to the county after the death of Pieter son of Gheront.

In 1355 Count William V confirmed Floris' possession of these lands, lordships, mills, and tithes.

At the time (after the surrender of Heusden) the count offered Floris to make a division of these goods.

In gratitude for this service with regard to the peace with Brabant, Floris became castellan and lord of the land of Heusden for life.

He was locked up, and in February 1358 his younger brother Albert I, Duke of Bavaria arrived in Holland to take up the regency.

The perpetrators fled to Heemskerk Castle, which the count's forces besieged from 4 December 1358 to 24 March 1359.

Afterwards, some Hook lords fled to Heusden Castle, where they were apparently received by Floris van Borselen.

[29] Shortly after, on 17 March 1359, the conflict between the Van Borselen clan and allies, and Middelburg was judged by Regent Albert.

[31] In late 1365 Floris van Borselen lord of Sint-Maartensdijk was appointed as Stadthouder of Zeeland.