In 1952 he left Latin America for Switzerland where he worked at the International Labour Organization. After he retired from the ILO he went on to teach at the University of Fribourg. He considered returning to Germany at times, but had difficulty securing a position and regaining German citizenship.
Peter Thullen was an enthusiastic Wandervogel and active in the Catholic youth movement and opposed the rise of Nazism. He at first studied in Italy on a grant, and was able to observe developments in Germany from abroad. He decided he would not return to Germany so long as Hitler remained in power.
After marriage, he moved to Quito, Ecuador with his wife. At the time he left for Ecuador he did not even know where Quito was located. His five children were all born during his stay in Ecuador.
He would later disapprove of the post-war regime of Konrad Adenauer as he felt it retained some of the "ills" of German nationalism.
Thullen, Peter (1931), "Zu den Abbildungen durch analytische Funktionen mehrerer komplexer Veraenderlichen Die Invarianz des Mittelpunktes von Kreiskoerpern", Mathematische Annalen104: 244–259, doi:10.1007/bf01457933