He was son of Cornelis van Aarsens, also a statesman. His talents commended him to the notice of Advocate Johan van Oldenbarnevelt, who sent him, at the age of 26 years, as a diplomatic agent of the states-general to the court of France. He took a considerable part in the negotiations of the Twelve Years' Truce in 1606.
His conduct of affairs having displeased the French king, he was recalled from his post by Oldenbarneveldt in 1616. Such was the hatred he henceforth conceived against his former benefactor, that he did his very utmost to effect Oldebarneveldt's ruin. He was one of the packed court of judges who in 1619 condemned the aged statesman to death. For his share in this judicial murder a deep stain rests on the memory of Aarssens.
He afterwards became the confidential counsellor of Maurice, Prince of Orange, and afterwards of Frederick Henry, Prince of Orange, in their conduct of the foreign affairs of the republic. He was sent on special embassies to Venice, Germany and England, and displayed so much diplomatic skill and finesse that Cardinal Richelieu ranked him among the three greatest politicians of his time.