James A. Garfield served an abbreviated term, as he was one of only four presidents to be assassinated, the other three being Abraham Lincoln, William McKinley, and John F. Kennedy. He was in office from March 1881 to September 1881.
Garfield had served in the Union army and then in the House of Representatives from 1863 to 1881. The Republicans were seeing trouble after the unpopular, corrupt administrations of Ulysses Grant and Rutherford Hayes. James Garfield was a "dark horse" nomination for the troubled party.
Garfield's term was deeply rooted in civil rights and reform: he reformed the post office, recommended a universal education system, and appointed many freedmen, such as Frederick Douglas, to prominent positions. However, due to his short term, he became one of the four 'lost presidents', the easily forgotten Commander-in-Chiefs following the Civil War. The lost presidents are Hayes, Garfield, Arthur, and Harrison.
Unfortunately, when Garfield and his family visited his old college, Williams College, in July, Charles J. Guiteau followed him. The assassin shot him twice, and the bullets led to infection and a weakening heart. Garfield finally passed away in September.
Guiteau had wanted to be a consul in Paris, and he believed his speech had helped Garfield win the presidency. Therefore, he believed he had earned such a position. When he was turned down for the position for being unqualified, and subsequently banned from the White House for being aggressive, Guiteau refused to accept his rejection gracefully and instead shot the President. Following Garfield's death, his Vice President, Chester Alan Arthur, took office.
Sources: History.com, Biography.com, Miller Center