Cornelis de Witt was a member of the old Dutch patrician family De Witt. He was born on 15 June 1623 in Dordrecht, Holland, Dutch Republic. He was the son of Jacob de Witt and the older brother of Johan, and also a close relative to the great Dutch regents Cornelis and his brother Andries de Graeff and their cousin Andries Bicker.

In 1650 he became burgomaster of Dordrecht and member of the States of Holland and West Friesland. He was afterwards appointed to the important post of ruwaard or governor of the land of Putten and bailiff of Beierland.

He associated himself closely with his greater brother, the Grand Pensionary, and supported him throughout his career with great ability and vigour. In 1667 he was the deputy chosen by the States of Holland to accompany Lieutenant - Admiral Michiel de Ruyter in his famous raid on the Medway. Cornelis de Witt on this occasion distinguished himself greatly by his coolness and intrepidity. He again accompanied De Ruyter in 1672 and took an honorable part in the great battle of Solebay against the united English and French fleets. Compelled by illness to leave the fleet, he found on his return to Dort that the Orange party were in the ascendancy, and he and his brother were the objects of popular suspicion and hatred. He was arrested on false accusations of treason, but did not confess despite heavy torture and was ultimately unlawfully condemned to be banished.

He was assassinated by the same carefully organised lynch mob that killed his brother on the day he was to be released, victim of a conspiracy by the Orangists Johan Kievit and Lieutenant - Admiral Cornelis Tromp. Both their bodies were horribly mutilated and their hearts were carved out to be exhibited as trophies. Today this is seen by the Dutch as the most shameful event in the history of Dutch politics.

Cornelis de Witt was married to Maria van Berckel (1632 – 1706). The couple had one daughter, Wilhelmina de Witt (1671 – 1702). She was married with her cousin (the son of Johan de Witt) Johan de Witt Jr. (1662 – 1701), secretary of Dordrecht.