[1]: 224–225 After he completed his studies in France he became secretary of Albert Joachimi, ambassador of the States-General of the Netherlands at the Court of St James's around 1625.
[1]: 224 After the death of stadtholder William II, Prince of Orange he played an important role in the States-Party revolution that inaugurated the First Stadtholderless Period in Dutch political history.
As a confidant of Grand Pensionary Johan de Witt he played a leading role in the Dutch embassy, together with Beverningh.
However, both took an oath on their innocence, and the prosecution came to nothing[1]: 224 After the treaty had been concluded Nieupoort remained behind in England as ambassador to the Commonwealth.
He was not employed in the negotiations of the Treaty of Breda (1667) after the Second Anglo-Dutch War (unlike his colleagues Beverningh and Jongestall).
[1]: 224 After the murder of Johan and Cornelis de Witt in August 1672, he was attacked by an Orangist mob in Schiedam and imprisoned, because of his States-Party affiliation.