Van der Capellen became a member of the States of Overijssel in 1772. It was the beginning of his political career. He described himself as a "born regent", but that did not prevent him from being an ardent champion of the Enlightenment ideals and a critic of the Dutch Old Regime.

For that reason, his opponents compared Van der Capellen's public appearance with the style of the English politician John Wilkes. Wilkes criticized the policy of King George III and his ministers, but became very popular. This was partly due to his strategic use of the political press. The press also became a powerful weapon in the hands of Van der Capellen and his patriot friends.

As a member of the States Van der Capellen applied himself to the abolition of the so-called "drostendiensten", which compelled the farmers of Overijssel to labour for a pittance a few days every year for the local magistrate, the Drost ("bailiff"). This relic from medieval times was wrongfully applied in his opinion.

In an address to the States in 1778, he pointed out that these rights had been officially abolished in 1631. He ensured his statement was distributed widely and free of charge among the farmers in the Twenthe region. The reverend François Adriaan van der Kemp was his assistant or ghostwriter.

In the end he won his case but his fellow members in the States excluded him from participation in their deliberations until November 1782.

Previously, he had fallen foul of the Stadholder prince William V of Orange and the members of the States when he proclaimed himself against expansion of the army and the fleet. When in 1776 the British King George III asked the Dutch whether he could borrow the Scotch Brigade (a unit of mercenaries in Dutch service) to be deployed in the war with the American Republic, he voted against.

His overt support for the American revolutionaries was not appreciated either. Nevertheless, he continued to dedicate himself to the American War of Independence. He regarded the American struggle as an example for Dutch patriots.

To lend support to his views he translated "Observations on Civil Liberty" by the Welshman Richard Price into Dutch. The book was an important inspiration to the American revolutionaries. The Dutch translation was banned in 1789, together with other patriot writings.

In 1782 Van der Capellen arranged a loan for the American cause. A consortium was set up by Nicolaas van Staphorst and Wilhelm Willink. Eventually an amount of two hundred thousand guilders was raised. He contributed twenty thousand guilders himself.

The year 1781 was to be the most important in his political career. In that year a pamphlet appeared entitled "Aan het Volk van Nederland" ("To the People of the Netherlands"). In this anonymous pamphlet the disadvantages of the hereditary stadholderate were explained. It should be replaced by a democratic society, based on popular sovereignty. Within a month after publication the pamphlet was banned. Besides, a substantial reward was offered to whoever would report the author. In spite of these measures it was illegally reprinted and distributed three times, and even translated into French, English and German. It was to exert a lasting influence on the democratic movement in the Netherlands.

Van der Capellen, the author of the pamphlet, was forced to enjoy its success in silence, as well as the fact that the authorities failed to identify him as the author. With unflagging zeal he continued to propagate the American cause. Partly thanks to his influence, John Adams and with him the young American Republic were recognized in 1782 by the States General.

He died in 1784. Even after his death he continued to rouse the emotions. A few years after he died his monument in Goor was blown up by Orangists.

The charismatic but controversial Dutch politician Pim Fortuyn (1948 – 2002) was deeply inspired by Van der Capellen. In 1992 Fortuyn wrote a "To the people of the Netherlands" too and declared himself as the successor of Van der Capellen who has to struggle against the political elite of his time.