Trefousse was born in Germany and emigrated to the United States in the 1930s. He graduated from New York City College in 1942. He then enlisted in the Army where he served as an intelligence officer in WW II, using his fluent German to interrogate German soldiers.

Trefousse focused his work on the history of the Reconstruction, and wrote books on Andrew Johnson and Rutherford B. Hayes. In the 1960s, he confronted the dominant historical narrative of the time regarding Reconstruction, arguing instead that Reconstruction was a failed attempt "to bring racial justice to the South." In total he authored over twenty books.

He gained media attention during Bill Clinton's impeachment for drawing comparisons to Johnson's impeachment.

Books

• The Radical Republicans: Lincoln's Vanguard for Racial Justice
• Andrew Johnson
• Impeachment of a President: Andrew Johnson, the Blacks, and Reconstruction
• Thaddeus Stevens: Nineteenth-Century Egalitarian
• Rutherford B. Hayes
• German and American Neutrality, 1939-1941
• Ben Butler: The South Called Him Beast!
• Carl Schurz: A Biography
• Pearl Harbor: The Continuing Controversy