The longest serving president was Franklin Delano Roosevelt who served 4 terms and a total of 12 years 1 month and 8 days as the president of the United States.n March 21, 1947 America passed the twenty-second amendment (Amendment XXII) to the United States Constitution and it was ratified by the requisite number of states on February 26, 1951 which limited the term any one person can serve as President.