In 1934, due to the rise of the Nazis, he immigrated first to Great Britain and then to the United States, where he joined the staff of Arthur Upham Pope at the Institute of Persian Art and Archaeology in New York. From 1937 to 1938, he taught his first class at the Institute of Fine Art, New York University. In 1938 he was appointed an associate professor at the University of Michigan.
In 1944, Ettinghausen left Michigan to join the Freer Gallery. The following year he married the art historian Elisabeth Sgalitzer. He also lectured at Princeton University. In 1961 he was appointed chief curator of the Freer. During his tenure at the Freer, he built the collection into one of the finest collections on Islamic art in the world. He wrote a book "Arab Painting: Treasures of Asia, Vol IV" published by Editions d'Art Albert Skira, Geneva in 1962.
In 1966, Ettinghausen left the Freer to become Hagop Kevorkian Professor of Islamic Art at the Institute of Fine Art, New York University. Together with the Middle East historian R. Bayly Winder he founded the Kevorkian Center the same year at NYU.
Three years later, he also became the Consultative Chairman of the Islamic Department of the Metropolitan Museum of Art. At the Metropolitan, he was instrumental in installing the galleries to their sensitive arrangement. His text, with Oleg Grabar, The Art and Architecture of Islam 650-1250 in the Pelican History of Art series, appeared posthumously in 1987.
Ettinghausen died of cancer in Mercer, New Jersey. The library in the Kevorkian Center is named in his honor.