By the time he finished school, Cornelissen had renounced his native religion of Catholicism and taught himself how to speak the Latin and English languages.

In the latter city, he joined a political club that agitated for universal suffrage, and began editing its newspaper Licht en Waarheid, publishing its first issue in May 1899.

He subsequently joined the Social Democratic League (SDB), becoming a member of its central council and co-editing its newspaper Recht voor Allen along with Ferdinand Domela Nieuwenhuis, together with whom he led the organisation's anti-parliamentary faction.

By 1891, Cornelissen was already advocating for syndicalism and began calling on revolutionary socialists and social anarchists to form a united front based on a shared anti-parliamentary viewpoint.

In France, Cornelissen focused on his work in journalism and economics, largely falling out of contact with the Dutch workers' movement.