In 1918, as World War I was coming to an end, he was drafted into the Austro-Hungarian army though he had not completed school, and sent to the Italian front.

He taught music, including harmony and counterpoint, for a year at Vassar College (1930–31), filling in for a professor on sabbatical.

At the same time he worked on a dissertation on the literary history of French opera, earning a doctorate degree from Cornell University in 1934.

In 1940, after Bartók fled Hungary during World War II, Lang arranged for Columbia to hire him as an ethnomusicologist.

As musicology was a nascent field at the time, Lang had a strong influence on its development, especially in the United States, and advised a number of students who would go on to become prominent musicologists, including James McKinnon, Dika Newlin, Joel Sachs, Rose Rosengard Subotnik, Richard Taruskin, Piero Weiss, and Neal Zaslaw.

He published a number of books, the most famous of which is Music in Western Civilization (1941), called "[a] model of scholarship and style"[6] by Will Durant.