Buchanan had two dogs--a Newfoundland named Lara and a toy terrier named Punch. He also kept a pair of bald eagles.

Multiple portraits show the six-foot Buchanan cocking his head to the left, as a defect in one of his eyes made him tilt his head "in a perpetual attitude of courteous deference and attentive interest." Modern ophthalmologists believe he may have suffered from exodeviation, a form of wandering eye.

Buchanan appointed one Justice, Nathan Clifford, to the Supreme Court of the United States.

The Utah War, also known as Buchanan's Blunder, was an armed confrontation between federal forces and Mormon settlers in the Utah Territory who had discouraged outsiders from settling in the Salt Lake City area and massacred a group of families from Arkansas. Believing the Latter-day Saints to be in open rebellion, Buchanan sent the army in 1857 to forcibly remove Brigham Young as governor. Young reacted by mustering a two-week expedition, destroying wagon trains, oxen, and other Army property. Eventually, a settlement was reached. The new governor took office, and Buchanan granted amnesty to all inhabitants who affirmed loyalty to the U.S. government.

Buchanan was dedicated to defending his actions prior to the Civil War, which erupted within two months of his retirement, and which some referred to as "Buchanan's War." He received threatening letters daily, and stores displayed Buchanan's likeness with the eyes inked red, a noose drawn around his neck and the word "TRAITOR" written across his forehead. Distraught by the vitriolic attacks levied against him, he soon began writing a public defense, in the form of his memoir Mr. Buchanan's Administration on the Eve of Rebellion, which was published in 1866.