Born in Amersfoort, Van Oldenbarnevelt studied law at several universities throughout Europe before settling in The Hague. Although just a moderate Calvinist, he supported William the Silent in his revolt against Spain, and fought in William's army.
He was a fierce opponent of the policies of the Earl of Leicester, the governor-general at the time, and instead favoured Maurice of Nassau, a son of William. Leicester left in 1587, leaving the military power in the Netherlands to Maurice.
In 1586, Van Oldenbarnevelt was made Land's Advocate of the province of Holland, an office he held for 32 years. Holland being the most important province of the Dutch Republic, and Van Oldenbarnevelt being a very able policitian, he became one of the most powerful men in the country.
On April 9, 1609, Van Oldenbarnevelt managed to negotiate a truce with Spain, much against the will of his former protege Maurice, even though the deal was favourable to the Netherlands.
The relationship between the two became worse when they ended up in opposing camps in an ongoing religious conflict in the Netherlands. Van Oldenbarnevelt supported the more libertine Arminians (or Remonstrants), while Maurice gave his support to the strict Calvinist Gomarists (or Contra-Remonstrants).
When Van Oldenbarnevelt proposed several measures which de facto called for independence of the province of Holland from the rest of the Republic, Maurice intervened, and Van Oldenbarnevelt was arrested on August 23, 1618 on accusation of treason.
A show trial followed, in which no real evidence was presented and in which several judges were known political enemies of Van Oldenbarnevelt, he was sentenced to death. He was executed at the Binnenhof in The Hague.