Which president had the longest military career?

There were three U.S. Presidents whose military careers spanned 40 years or more. Dwight David Eisenhower had the longest military career of any U.S. President. His service took place in two stages. The dates of the first stage were 1915-1952. His career ended with his retirement from the military in order to conduct two successful campaigns for the presidency, and to participate in two inaugurations to presidential office. But it was re-opened with 37th U.S. President John Fitzgerald Kennedy's respectful and unprecedented full restoration of the 36th U.S. President to his previous rank as Five-Star General in the U.S. Army. So the date of the second stage began in 1961, and ended with his death in 1969.

Seventh U.S. President Andrew Jackson had the second-longest military career. His military career spanned service, at 13, in the Revolutionary War to military governorship of the Florida Territory, in 1821, after the end of the First Seminole War. So the dates of President Jackson's military career were 1780-1822.

Twelfth U.S. President Zachary Taylor had the third-longest military career. His military career spanned commission [from his cousin, subsequent 4th U.S. President James Madison] as first lieutenant of the 7th Infantry Regiment, to the Mexican-American War. So the dates of President Taylor's military career were 1808-1848.