Jacob was a member of the old Dutch patrician family De Witt. He studied law at Leiden University and obtained a law degree there. In 1618, he became treasurer of the Synod of Dort where he held several positions in public service, for instance serving as burgomaster six times. He also served as emissary to Sweden along with Andries Bicker.

He was a member of the States of Holland and as such opposed to William II, Prince of Orange, who was the stadtholder of Holland, Zeeland, Utrecht, Guelders and Overijssel. Together with the republican - minded brothers Cornelis and Andries de Graeff and their cousins Andries and Cornelis Bicker, De Witt supported the Peace of Münster and, in May 1650 filed a proposal to reduce the size of the army which caused political friction.

The affair led to a coup of the stadtholder who, in secret, staged an attack on Amsterdam. On 30 June 1650, De Witt and five other prominent members of the States were arrested at the Binnenhof in The Hague. The burgomasters of Delft, Hoorn, Medemblik, Haarlem, and Dordrecht were kept prisoner in Castle Loevestein. On 17 August they were released after having reversed the reduction of the army size.

In 1657 he moved to The Hague, but after the murder of his sons on 20 August 1672, on which he was brought to safety, he avoided The Hague, spent a short time in Dordrecht where he died in on 10 January 1674.