Alfred Pleasonton was born in Washington, D.C. on July 7, 1824, the son of Stephen Pleasonton and Mary Hopkins. In 1844 he graduated seventh in his class at the United States Military Academy. He served in the Mexican War, the Indian wars on the frontier, and was Adjutant-General in the Seminole War.

In 1861 he was appointed Major. He distinguished himself in the Pennisular Campaign (1862) and was appointed Brigadier General of Volunteers that same year.

He fought at Antietam and Fredericksburg, and his stand against Stonewall Jackson at Chancellorsville averted a total Union defeat. He commanded the Union cavalry at Brandy Station and in the ensuing Gettysburg campaign, as well as later engagements. After being transferred to Missouri, Pleasonton defeated General Sterling Price at Westport and Marais des Cygnes (1864), ending the last Confederate threat in the West.

In 1865 he was brevetted Brigadier General of the United States Army. He became United States collector of internal revenue in 1868 and a few years later was appointed president of the Terre Haute and Cincinnati Railroad.

Alfred Pleasonton died at Washington, D.C. on February 17, 1897 and was buried in Congressional Cemetery.