All that is known of his early years is that he was born in Amsterdam, the son of Pieter Rijnst (ca.1510–1574), soap Boiler from the later patrician Reynst family, and Trijn Sijverts.

In 1599, as a merchant and ship-owner, he became a founder-member and administrator of the Nieuwe or Brabantsche Compagnie which, in 1600, became the Vereenighde Company of Amsterdam.

On the request of his elders in the college of the Heren XVII (17 men), he became Governor-general of the Dutch East Indies in 1613 and left with 9 ships.

There is a street named after him in The Hague, Gerard Reijnststraat, which is situated very close to the area which the Allies mistakenly bombed during World War II.

His daughter Weijntje became the mother of the merchant Isaak Isaaksz Coymans (1622–1673), one of the founders of the Danish West India Company.