The roots of isolationism go back to the founding of the country, with the idea with the idea of escaping to America from prosecution. There is the assumption about America from it's inception that it will be a better place than where one has come from. Founding fathers such as Washington noted that Europe's interests had "none of very remote relation" to those of the United States.
To suggest that America had held an isolationist stance before 1890 is putting things too simply, however. Overseas bases were built in the Carribean and the Pacific. America engaged in the War of 1812, the Mexican War and the Spanish-American war. The latter two of these specifically had expansionist ideals, either foreign or within the continent.
The "closing of the frontier" is also commonly seen as causing a shift in foreign policy, as it created anxieties around the continued progression of American ideals, which would now shift to be continued outside of the borders of the nation. Yet fundamental socioeconomic, political and economic changes brought about by improved transportation and communication reduced isolationism through a larger role of foreign trade and shipping. The decline of rural America lead to changing ideas about isolationism and traditional policies held by the public. Indeed, Walter Russell Mead argues that there has been a long tradition of this international trade and so rejects ideas of American being traditionally isolationist due to this.
The limited involvement in World War I highlights the shifting nature of isolationist policies. America was not yet fully ready to commit, with figures such as William Jennings Bryan fighting hard against American involvement. The U.S. also did not join the League of Nations again showing involvement but a lack of full commitment to foreign affairs.
Throughout the early 20th century the term 'isolationist' continued to be used as a denigrating term despite many Americans opposing the country becoming involved in the affairs of Europe and Asia. Isolationism was most strongly held in the small towns of America, as it had been for many decades. Irish-Americans and German-Americans were some of the staunchest followers of isolationist policies. They viewed the urban business and factories of the east with disdain and denounced the "elites for involvement in European affairs."
1940 can be seen as a final turning point for American isolationism with the battle of Britain and the German military's successes forcing the hand of the American government to lend significant help to the Allies. Peal Harbor would follow the next year, changing the minds of any who still believed in American isolationist policies.
Citations:
Walter Russell Mead, The American Foreign Policy Legacy, Foreign Affairs , Vol. 81, No. 1 (Jan. - Feb., 2002)
Wayne. S. Cole, Roosevelt and the Isolationists, 1932-194,Univ. of Nebraska, 1983
John Milton Cooper, The Vanity of Power: American Isolationism and the First World War, 1914-1917, Greenwood Publishing Corp., 1969
I, and many historians, would question the notion that the United States was isolationist prior to the 1890's. Within the first 30 years of Independence, the United States was involved in no less than four separate wars against powers of the old world and the domestic political conflicts of the era are largely defined by foreign policy. The Monroe Doctrine, participation in the Panama Congress of 1826, military expeditions into eastern Asia, Mexican-American War, Clayton-Bulwer treaty, picking up of Kossuth from Hungary, attempted annexation of Santo-Domingo in the post war of 1812 era all also point towards a United States that was much more involved in international affairs than some would believe.