Xantus (the aristocratic title de Vesey was an affectation, of which he had several variations) was born Xántus János, in Csokonya, Somogy, Hungary.
[1] In the U.S. he pursued a variety of occupations, including bookseller, druggist, a teacher, and hospital steward in the U.S. Army.
In the Army he met Dr. William Alexander Hammond, a collector for the noted zoologist Spencer Fullerton Baird.
Working under Hammond as an assistant surgeon, he soon developed an interest in natural history and became a gifted collector himself.
In 1860 he was stationed as a tidal observer at Cabo San Lucas, on the southern tip of the Baja California peninsula, where he collected natural history specimens for the United States National Museum.