Disinterested in practicing law, he began working as a journalist in Zagreb. In 1939 he went to Argentina to write about former Croates who had left Yugoslavia to settle in South America. He stayed there for the next 15 years as a journalist and explorer. In 1944 he was part of a trekking party that climbed to the top of highest mountain in the western hemisphere Aconcagua , near Argentina. He used this experience to write a novel (story) in Spanish about Aconcagua, which became a best seller there. In 1946 he undertook the first of two expeditions into the Amazon jungle, the first of which produced a popular book, (Por Tierra De Indios). His companion from that expedition, Mary Reznik—whom he later married—was the first white woman to have penetrated that deeply into the Brazilian jungle. Together they returned to the Amazon in 1948, after which he penned "Where Civilization Ends" (Donde La Civilizacion Termina). The marriage produced a son, Diego. In 1954 Sekelj returned to Yugoslavia and settled in Belgrade, although he still traveled extensively. In 1962 he traveled through Africa. He married Erzsebet Sekelj, a young woman he met on a trip through Hungary. From 1972 he lived in Subotica (Vojvodina), now in Serbia. He was in charge of a museum in Subotica. He died in Subotica, and is buried there.