Who is the longest-serving president in American history?

I thought there was a catch to this question, but the longest serving president in U.S. history was Franklin D. Roosevelt who was elected to four terms as U.S. president. His first term began on March 4, 1933 and lasted until his death on April 12, 1945. That means that he served a total of 12 years, one month and 8 days as president of the United States. Other men have had long careers serving in government before they finally became president, but FDR served the longest as President of the United States.