See the biographical essays in Freedom and Reform, ed. by H. M. Hyman and L. W. Levy (1967).
Commager studied Danish history, and wrote his Ph.D. dissertation on the Danish philosopher and reformer Johann Struensee. Under the influence of his mentor at Chicago, the constitutional historian Andrew C. McLaughlin, Commager shifted his research and teaching interests to American history. He was coauthor, with Samuel Eliot Morison, of the widely-used history text The Growth of the American Republic (1930; 1937; 1942; 1950, 1962; 1969; 7th ed., with William E. Leuchtenburg, 1980; abridged editions in 1980 and 1983 under the title Concise History of the American Republic). His anthology, Documents of American History (1938), reaching its tenth edition (coedited with his former student Milton Cantor) in 1988, half a century after its first appearance, remains a standard reference work. His two documentary histories, The Blue and the Gray and The Spirit of Seventy-Six (the latter co edited with his longtime friend and Columbia colleague Richard B. Morris), treat the Civil War and the American Revolution, respectively, as seen by participants.
With Richard B. Morris, he also co edited the New American Nation Series, a multi-volume collaborative history of the United States under whose aegis appeared many significant and prize-winning works of historical scholarship.
Commager's first solo book was his 1936 biography, Theodore Parker: Yankee Crusader, a life of the Unitarian minister, Transcendentalist, reformer, and abolitionist Theodore Parker; it was reissued in 1960, along with a volume edited by Commager collecting the best of Parker's voluminous writings. His most characteristic books were his 1950 monograph The American Mind: An Interpretation of American Character Thought since the 1880s; and his 1977 study The Empire of Reason: How Europe Imagined and America Realized the Enlightenment. As these books suggest, he was principally an intellectual and cultural historian, but also worked in the fields of constitutional and political history. His work on this subject includes his controversial 1943 series of lectures, Majority Rule and Minority Rights.
Commager was an ardent defender of the Constitution and the Bill of Rights. He opposed McCarthyism in the 1940s and 1950s, the U.S. war in Vietnam (on constitutional grounds), and what he saw as the rampant illegalities and unconstitutionalities perpetrated by the administration of Richard M. Nixon. One favorite cause was his campaign to point out that, because the budget of the Central Intelligence Agency is classified, it violates the requirement of Article I of the U.S. Constitution that no moneys can be spent by the federal government except those specifically appropriated by Congress.
Commager also wrote many essays on history for popular magazines and newspapers (many of them collected in such books as Freedom, Loyalty, Dissent; The Search for a Usable Past and Other Essays in Historiography; Freedom and Order: A Commentary on the American Political Scene; The Commonwealth of Learning; The Defeat of America: War, Presidential Power and the National Character; and Jefferson, Nationalism, and the Enlightenment. He frequently was interviewed on television news programs and public-affairs documentaries to provide historical perspective on such events as the Apollo XI moon landing and the Watergate crisis.
Commager insisted that historians must write not only for one another but for a wider audience.
Commager once said about teaching, "What every college must do is hold up before the young the spectacle of greatness."
On July 14, 1979he married his second wife, the former Mary Powlesland, a professor in Latin American studies, in Linton, England. With her he lived out the rest of his days. Commager died of pneumonia at the age of ninety-five under Mary's care at their home in Amherst.
In the Spring 2004 edition of History of Education Quarterly, Jonathan Zimmerman wrote,
The authors finally removed the passage in the 1962 version (fifth edition) of their text book. The passage echoes the thesis of American Negro Slavery by Ulrich Bonnell Phillips. This view, popularized by most white historians until the mid twentieth century, relied on the one-sided personal records of slave-owners and portrayed slavery as a mainly benign institution.
"The Phillips school of slavery historiography was not limited to the South or to a faction within the historical profession; as recently as 1950, for instance, Samuel Eliot Morison and Henry Steele Commager, of Harvard and Columbia Universities respectively, propagated the traditional interpretation in one of the leading college textbooks of the era," according to the American Social History Project at the City University of New York.
Pulitzer Prize winning historian Leon F. Litwack found the widely used textbook offensive saying, "The textbook was my first confrontation with history. I asked my 11th grade teacher for the opportunity to respond to the textbook’s version of Reconstruction, to what I thought were distortions and racial biases.(I had already read Howard Fast’s Freedom Road.) The research led me to the library—and to W.E.B. Du Bois’s Black Reconstruction, with that intriguing subtitle: An Essay Toward a History of the Part which Black Folk Played in the Attempt to Reconstruct Democracy in America, 1860–1880. Armed with that book, I presented what I thought to be a persuasive rebuttal of the textbook.
As co-editors of The New Americn Nation Series, Commager and Richard B. Morris cowrote the introduction to Eric Foner's Reconstruction: America's Unfinished Revolution, 1863-1877, the book that, they concluded, is a "scholarly convincing Reconstruction of what is indubitably the most controversial chapter in our history."