His father fled to Dordrecht in 1595 to escape from the Inquisition and became prosperous in business.

Liege was then, as now, a great centre of the iron industry; and after his father's death Louis de Geer in 1615 removed to Amsterdam, where he became a merchant in all kinds of iron and copper goods, more especially of ordnance and fire-arms.

In close alliance with him, though not in partnership, was his brother-in-law, Elias Trip, the head of a firm reputed to have the most extensive business in iron-ware and weapons in the Netherlands.

The commanding abilities of de Geer soon gave to the two firms, which continued to work harmoniously together as a family concern, a complete supremacy in the class of wares in which they dealt.
At this time the chief supply of iron and copper ore came from Sweden; and in 1616 de Geer was sent on a mission by the States-General to that country to negotiate for a supply of these raw materials for the forging of ordnance.

This mission had important results, for it was the first step towards bringing about those close relations between Sweden and the United Provinces which were to subsist throughout the whole of the Thirty Years' War.