Hartz graduated in 1940, spent a year traveling abroad on a fellowship, then returned to Harvard as a teaching fellow in 1942. He earned his doctorate in 1946 and became a full professor of government in 1956. Hartz was known at Harvard for his talented and charismatic teaching. He retired in 1974 due to ill health and spent his last years living in London, New Delhi, New York, then Istanbul, where he died.
Hartz is best known for his classic book The Liberal Tradition in America (1955) which presented an original view of America's past that sought to explain its conspicuous absence of ideologies. Hartz argued that American political development occurs within the context of an enduring, underlying Lockean liberal consensus, which has shaped and narrowed the landscape of possibilities for U.S. political thought and behavior. He attributed the triumph of the liberal worldview in America to its lack of a feudal past, and thus the absence of a struggle to overcome a conservative internal order; to its vast resources and open space; and to the liberal values of the original settlers, who represented only a narrow middle-class slice of European society. Hartz was chiefly concerned with explaining the failure of socialism to become established in America, and believed that Americans' pervasive, unthinking consensual acceptance of classic liberalism was the major barrier. Hartz thus firmly rejected Marxist ideas about the inevitability of class struggle.
In The Founding of New Societies (1964), Hartz developed the idea that the nations that developed from settler colonies were European "fragments" that in a sense froze the class structure and underlying ideology prevalent in the mother country at the time of their foundation, not experiencing the further evolution experienced in Europe. He considered Latin America and French Canada to be fragments of feudal Europe, the United States, English Canada, and Dutch South Africa to be liberal fragments, and Australia and English South Africa to be "radical" fragments (incorporating the non-Socialist working class radicalism of early 19th century Britain).