Parrington is best remembered as the author of Main Currents in American Thought, a politics-centered three-volume history of American letters from colonial times, postulating a sharp divide between the elitist Hamiltonian current and its populist Jeffersonian opponents, and making clear Parrington's own identification with the latter.
Parrington defended the doctrine of state sovereignty, and sought to disassociate it from the cause of slavery. He wrote that the association of those two causes had proven "disastrous to American democracy," removing the last brake on the growth of corporate power, because in the gilded age the federal government had shielded capitalists from local and state regulation.
Main Currents won the Pulitzer Prize for History in 1928, and was for many years one of the most influential books for American historians. Reising (1989) shows the book dominated literary and cultural criticism from 1927 through the early 1950s. Crowe (1977) argues that it was "was the "Summa Theologica of Progressive history." Progressive history was a set of related assumptions and attitudes, which inspired the first great flowering of professional American scholarship in history. These historians saw economic and geographical forces as primary and, and saw ideas as merely instruments. They regarded many dominant concepts and interpretations as masks for deeper realities.
Reinitz (1977) stresses Parrington's heavy use of historical irony, which occurs when the consequences of an action emerge contrary to the original intentions of the actors. Parrington represented the Progressive School of historians which stressed the duality of good versus evil in the American past. Yet, in his final volume of Main Currents he concluded that the Jeffersonian farmer, the Progressives' traditional democratic hero, had joined forces with the greedy business community to produce a destructive form of capitalism which culminated in the 1920s.
Parrington was the second head coach of the University of Oklahoma football team, where he was the first OU faculty member to officially hold the position. He is credited with bringing a Harvard style of play and better organization to the OU football program. During his four year stretch from 1897 to 1900 Parrington's teams played only twelve games, with 9 wins, 2 losses and 1 tie. Parrington's span as head football coach was the longest of any of Oklahoma's first 5 coaches.