This cousin of the Prince of Orange was endowed with brilliant qualities, and during the seven years of his governorship he extended the Dutch dominion from the Rio Grande in the south to the island of Maranhao on the north and to a considerable distance inland, indeed over the larger part of seven out of the fourteen captaincies into which Portuguese Brazil was divided.

On his arrival, by a wise policy of statesmanlike conciliation, he contrived to secure the goodwill of the Portuguese planters, who, though not loving the Dutch heretics, hated them less than their Spanish oppressors, and also of the Jews, who were numerous in the conquered territory.

Under his rule the Reciff as the seat of the Dutch government was beautified and enlarged; many fine buildings and gardens adorned it, and the harbour made commodious for commerce with rows of warehouses and ample docks.

To the new capital he gave the name of Mauritsstad.
During the earlier part of his governor-generalship Joan Maurice was called upon to face a really great danger.

The year 1639 was to witness what was to be the last great effort (before the Portuguese revolt) of the still undivided Spanish monarchy for supremacy at sea.