In 1934 he moved to Geneva, Switzerland. He was Jewish, and Vienna wasn't safe any longer. He married Margit Sereny-Herzfeld in 1938, at the relatively advanced age of 56. In 1940, when the Nazis invaded France, Margit and Ludwig left Geneva, emigrated to the United States, and settled in New York.
Von Mises became a naturalized U.S. citizen in 1946. Although he never received the professional recognition he undoubtedly deserved, he did spend many years teaching economics at the Foundation for Economic Education, and at New York University. He died October 10, 1973, at the age of 92. He continued to write and to lead seminars up until his ninetieth birthday.
Mises' masterpiece, Human Action, was first published in 1949. Although he was a native German speaker, he wrote this book in English.