While in secondary school, she worked as a volunteer at the IVN (Instituut voor Natuurbeschermingseducatie - Institute for Conservation Education).
Van Velzen was involved in direct action for disarmament at Faslane Naval Base in Scotland, where she was arrested after going aboard a nuclear submarine with a hammer to 'dismantle' it.
During the Kosovo War, she organised a walk from the International Criminal Court in The Hague to NATO headquarters in Brussels.
In the summer of the same year, she began work as an employee with the Socialist Party group in the House of Representatives, where she was concerned with defence, agriculture and food safety.
Her retirement pay was terminated after the TV programme PowNews revealed that she was staying abroad for a prolonged period and thus could apply elsewhere.