Esplorò il Sudamerica assieme alla moglie. Specie negli anni tra il 1940 e il 1965 pubblicò un gran numero di libri sulle antiche popolazioni degli Inca, dei Maya e degli Aztechi.
Nei primi anni '50 intraprese un'esplorazione di due anni in Perù alla ricerca delle antiche strade Inca, riuscendo a trovare l'unico ponte sospeso Inca giunto fino a noi.
Von Hagen, negli ultimi anni della sua vita, si era stabilito in Italia, dapprima a Trevignano Romano e poi in provincia di Arezzo: si interessò anche di antichità italiane.
Morì a sua casa di Montegonzi, frazione di Cavriglia in provincia di Arezzo, nella notte tra il 7 e l'8 marzo 1985 [1].
Sua figlia, Adriana von Hagen, è co-direttore di un museo a Leymebamba (Perù).
Alexander von Humboldt
Off With Their Heads (1937)
Ecuador the Unknown (1938)
The Tsátchela Indians of Western Ecuador (1939)
The Galapagos Islands and Charles Darwin: Notes on an Exhibition at the Bancroft Library (1939)
Quetzal Quest - The Story of the Capture of the Quetzal, the Sacred Bird of the Aztec and the Mayas (1939)
Jungle in the Clouds (1940)
Riches of South America (New World Neighbors) (1941)
Riches of Central America (New World Neighbors) (1942)
Paper and Civilization (Scientific Monthly, vol. 57, 1943)
Mexican Papermaking Plants (Journal of the New York Botanical Gardens, vol. 44, 1943)
The Jicaque Indians of Honduras (1943)
The Aztec and Maya Papermakers (1943)
South America Called Them: Explorations of the Great Naturalists: La Condamine, Humbolt, Darwin, Spruce (1945)
South American Zoo (1946)
F.Catherwood 1799-1854 - Architect-Explorer of Two Worlds (1946)
Maya Explorer: John Lloyd Stephens and the Lost Cities of Central America and Yucatán (1947)
The Green World of the Naturalists - A Treasury of Five Centuries of Natural History in South America (1948)
A Guide to Lima, the Capital of Peru (Guides to Peru) (1949)
A Guide to Cusco (Guides to Peru) (1949)
A Guide to Sacsahuaman: The Fortress of Cusco (Guides to Peru) (1949)
Ecuador and the Galápagos Islands: A History (1949)
Frederick Catherwood, Architect (1950)
Huancayo and Ayacucho (His Guide to Peru) (1950)
A Guide to Guayaquil (Guides to Ecuador) (1950)
A Guide to St Vincent (1950)
A Guide to Machu Picchu (Guides to Peru) (1952)
The Four Seasons Of Manuela. A Biography. The Love Story of Manuela Sàenz and Simòn Bolivar (1952)
Highway of the Sun (1955) - about an expedition of discovery of the ancient roads of the Inca
A Guide to Cusco and Machu Picchu (Guides to Peru) (1956)
Realm of the Incas (1957)
The Aztec: Man and Tribe (1958)
A Guide to Sacsahuaman: Ollontay-Tambo and Pisac (1958)
The World of the Maya (1960)
Maya, Land of the Turkey and the Deer (1960)
Ancient Sun Kingdoms of the Americas: Aztec, Maya, Inca (1961) [Antichi imperi del sole nelle Americhe, Mondadori, 1963]
The Incas: People of the Sun (1961)
A Chronological Chart of Pre-Columbian Indian Cultures of the Americas and World Events (1962)
The Desert Kingdoms of Peru (1965)
Roman Roads (1966)
Roads that Led to Rome (1967)
The Gold of El Dorado: The Quest for the Golden Man (1968)
The Incas of Pedro De Cieza De Leon (1970)
Search for the Maya: The Story of Stephens and Catherwood (1973)
The Germanic People in America (1976)
The Royal Road of the Inca (1976) [La strada reale degli Incas, Rizzoli, 1977]