His writings on Law and Constitutionalism made him one the world's leading political scientists in the post-World War II period. He is one of the most influential scholars of Totalitarianism.
In the 1920s, while a student in the US, Carl founded (and was President of) of the German Academic Exchange Service, through which he first met the love of his life, Lenore Pelham, also a writer and at the time a student at Rockford College outside of Chicago. The two later married. In 1926 he was appointed as a lecturer in Government at Harvard University. He received his Ph.D. from Heidelberg in 1930. When Hitler came to power he decided to remain in the United States and become a naturalized citizen. He was appointed Professor of Government at Harvard in 1936.
Friedrich's main areas of thought were the problems of leadership and bureaucracy in government, public administration, and comparative political institutions. An extremely popular lecturer, Friedrich also wrote prolifically, producing thirty-one volumes on political history, government, and philosophy and editing another twenty-two (then the second most in Harvard's long history). In the 1930s, Professor Friedrich was also a leading activist, with a student of his, the then-unknown David Riesman by his side, in efforts to repatriate Jewish scholars, lawyers, and journalists from Nazi Germany and other Fascist regimes to the United States. One that he helped, the pianist Rudolf Serkin, he convinced to give a concert at his farm in Brattleboro, Vermont, an event which led to the establishment of the Marlboro Music Festival.
An expert on German Constitutional Law and it the conditions surrounding its breakdown, Friedrich supported representative democracy. He strongly opposed direct democracy, however, particularly the use (or misuse) of referendums as leading to totalitarianism. He stressed the necessity for maintaining the rule of law, supplemented by a strong infrastructure of civil institutions, and was highly suspicious of grass-roots popular movements.
During World War II, Friedrich helped found the School of Overseas Administration to train officers for military work abroad and served as its director from 1943-46. He also served on the Executive Committee of the Council for Democracy, concerned with convincing the American people of the necessity for fighting totalitarianism and with strengthening national morale.
Friedrich, who was arguably the most knowledgeable scholar in his field (of German Constitutional history) of his time, was endowed with a healthy self regard. Indeed some of his colleagues at Harvard regarded him as a "somewhat hybristic person who was overly confident of his own ablities
Friedrich was the author of an article "Poison in Our System" for the June 1941 issue of the Atlantic Monthly, criticizing Songs For John Doe, an album of songs against Roosevelt's peacetime draft (issued in May, 1941, before Hitler's Germany had declared war on us), by the Almanac Singers, who included the then twenty-one-year-old Pete Seeger, performing under the pseudonymn 'Pete Bowers'. Friedrich was apparently as alarmed by the potential for uncontrolled spread of such topical songs as he was by their (fairly innocuous by our standards) content and opined that "mere" legal suppression would be an inadequate antidote, calling for the establishment of civilian pressure groups to conduct a cultural Freikorps as a countermeasure:
These recordings are distributed under the innocuous appeal: "Sing out for Peace". Yet they are strictly subversive and illegal. . . The three records sell for one dollar and you are asked to "play them in your home, play them in your union hall, take them back to your people." Probably some of these songs fall under the criminal provisions of the Selective Service Act, and to that extent it is a matter for the Attorney-General. But you never can handle situations of this kind democratically by mere suppression. Unless civic groups and individuals will make a determined effort to counteract such appeals by equally effective methods, democratic morale will decline.
From 1946-48 Professor Friedrich served as Constitutional and Governmental Affairs Adviser to the Military Governor of Germany, General Lucius D. Clay. He advised the American military on the denazification of Occupied Germany and participated in work leading to the drafting of the West German Basic Law and the creation of Germany's States' constitutions. He later advised on the constitutions of Puerto Rico, the Virgin Islands, and Israel, among others. Between 1955-71 Friedrich was Eaton Professor of the Science of Government at Harvard University and Professor of Political Science at the University of Heidelberg from 1956-66. He taught alternately at Harvard and Heidelberg, until his retirement in 1971. He later taught at the University of Manchester and Duke University, among others. He also served as president of the American Political Science Association in 1962 and of the International Political Science Association from 1967-70. In 1967, Friedrich was awarded the Knight Commander's Cross of the German Order of Merit by the President of the Federal Republic of Germany.
Professor Friedrich's many students included such noted political theorists as Judith Shklar, Benjamin Barber, and Zbigniew Brzezinski.