Hartz was born in Youngstown, Ohio, the son of Russian Jewish immigrants, but grew up in Omaha, Nebraska. After graduating from Technical High School in Omaha, he attended Harvard University, financed partly by a scholarship from the Omaha World Herald.
Hartz graduated in 1940, spent a year traveling abroad on a fellowship, and 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 for ill health and spent his last years living in London, New Delhi, New York City, then Istanbul, where he died of an epileptic seizure.
Hartz is best known for his classic book The Liberal Tradition in America (1955), which presented a 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 he believed that Americans' pervasive, unthinking consensual acceptance of classic liberalism was the major barrier.
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 and did not experience 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 nonsocialist working class radicalism of Britain in the early 19th century).
Hartz led a normal life until a sudden unexplained emotional disturbance changed his entire personality in 1971. He refused all medical help. He divorced in 1972, rejected all his friends, and feuded intensely with students, faculty and administrators. In 1974 he resigned from Harvard, but his scholarly skills and interests remained strong.
In 1956, the American Political Science Association awarded Hartz its Woodrow Wilson Prize for The Liberal Tradition in America, and in 1977 gave him its Lippincott Prize, designed to honor scholarly works of enduring importance. The book remains a key text in the political science graduate curriculum in American politics in universities today, in part because of the extensive, longrunning criticism and commentary that Hartz's ideas have generated.
Hartz's fragment thesis was disseminated and elaborated upon for the Canadian context by Gad Horowitz, in his essay "Conservatism, Liberalism and Socialism in Canada: An Interpretation" (1966). Horowitz's influential interpretation was still actively debated into the 21st century in Canadian political theory.
In Australia, Hartz's fragment thesis "received respectful attention, but ... did not win assent or committed followers", according to historian John Hirst. It was applied to early colonial history by feminist historian Miriam Dixson in The Real Matilda (1976), in which she traced gender relations in colonial New South Wales to the culture of the proletarian fragment identified by Hartz. In 1973, the Australian Economic History Review dedicated an issue to analysis of Hartz's theory.