It was Gravesande who made the decision that agricultural development should move towards Demerara. The fertility of the soil and the depth of the river helped him to make this decision. From 1746, he began to grant land on the banks of the Demerara River for sugar cultivation and within two years there were 18 plantations which were established. Settlements were growing at such a rapid pace that Gravesande recommended to the Directors of the Zeeland Chamber that a separate commander for the Demerara River should be appointed.

This recommendation was accepted, and in 1752, Gravesande himself was appointed Director-General of Essequibo and Demerara, while his son Johnathan, was appointed Commander of Demerara. Johnathan established the capital of Demerara on the island of Borsselen, located about 25 miles upriver. Here the Secretary's office, the Commander's house, a small fort and barracks for soldiers were built.

Johnathan Gravesande had hoped to encourage settlements around Borsselen, but new English settlers, who came in relatively large numbers, opted to settle and cultivate lands the banks of the river near to the Atlantic coast. In 1748, Laurens Gravesande had erected a guard house, or brandwagt, near the mouth of the river on its right bank (where Georgetown is today), and this provided protection to the settlements and plantations there. By 1763, English setters formed the majority of the population of Demerara and they owned roughly one-third of the existing plantations. They were also the first to introduce water-driven sugar mills which helped their plantations to show large profits.

By 1770, development in Demerara had far surpassed that in Essequibo. Four years earlier, a Dutch bank in Amsterdam had started to provide credit finance to sugar planters who seized the opportunity to increase their investments. The result was that while Essequibo's sugar plantations increased from 68 to 74, those of Demerara expanded from 93 to 130.

Johnathan Gravesande died in 1761. His brother-in-law, Laurens van Bercheyck, a land surveyor succeeded him to the post. He was credited in 1763, during the Berbice Slave Rebellion, for establishing an alliance with the Amerindians of Demerara to prevent the rebelling Berbice slaves from crossing into Demerara. He died in 1765 and his successor Cornelius van den Heuvel, a planter, proved to be very inefficient. Since he and the elder Gravesande were not on friendly terms, the administration of Demerara suffered. When some of the Demerara planters suggested that the capital of Demerara should be moved from Borsselen Island to the junction of the Hoobaboo Creek and the Demerara River, he opposed it because he owned estates near to the island. Finally, in 1770 he resigned from the post and Paulus van Schuylenburg was appointed to the position. The planters tried to get him to move the capital, but he also refused.

Gravesande himself resigned as Director-General of Essequibo-Demerara in 1772 and was succeeded by George Hendrik Trotz. Three years later on 14 August 1775 Gravesande died at his plantation, Soesdyke. Though it is believed that he was buried on Fort island, his grave has not been found.