Born in Baltimore, Maryland, he was a nephew by marriage of President John Quincy Adams, during whose administration Buchanan received an appointment to West Point. He graduated with a brevet to 2nd Lieutenant, 4th Infantry, July 1, 1830 and commission as 2nd Lieutenant, 4th Infantry. He served in garrison at Baton Rouge, LA, 1831-1832 and at Ft. Crawford, WI, 1832. He was in the Black Hawk War, 1832, being in command of the gunboats on the Wisconsin River during the Battle of Bad Axe River, August 2, 1832. Thereafter he was stationed at Ft. Armstrong, IL, 1832; Baton Rouge, LA, 1832-1833; and on Recruiting Service, 1833. He was in garrison at Baton Rouge, LA, 1833-1834 and New Orleans, LA, 1834-1835.

Buchanan served as Adjutant, 4th Infantry, at Regimental headquarters, February 4, 1835-November 30, 1838. He was in the Florida War against the Seminole Indians, 1836-1838, being engaged on Staff duty, in the Skirmishes at Campo Izard, February 27-29 and March 5, 1836. He was promoted to 1st Lieutenant, 4th Infantry, March 16, 1836. He was engaged in the Combat of Oloklikaha, March 31, 1836 and the Battle of Okeechobee, December 25, 1837. He served in the Cherokee Nation, 1838, while transferring the Indians to the West and was on Commissary Duty, at Louisville, KY, 1838-1839. He was promoted to Captain, 4th Infantry, November 1, 1838.

He was on Frontier Duty at Ft. Gibson, Indian Territory, 1839-1840 and involved in the opening of the military road to Ft. Smith, AR, 1840- 1841. He was in the Florida War against the Seminole Indians, 1841- 1842; on Frontier Duty at Jefferson Barracks, MO, 1842-1844, at Camp Salubrity, at Natchitoches, LA, 1844-1845 and in the Military Occupation of Texas, 1845-1846.

In the War with Mexico, Buchanan was engaged in the Battle of Palo Alto, May 8,1846 and the Battle of Resaca-de-la-Palma, May 9, 1846. He was breveted to Major, May 9, 1846, for gallant and distinguished services in the Battles of Palo Alto and Resaca-de-la-Palma.

He was engaged in the Battle of Monterey, September 21-23, 1846. Buchanan was commissioned a Lieutenant Colonel, Battalion of Maryland Volunteers, November 25, 1846 and in command of Battalion, November 25, 1846-May 30, 1847.

He was engaged in the Skirmishes of Paso de Ovejas, June 20, near Plan del Rio, June 22, and near El Pinal, July 5, 1847; the Capture of San Antonio, August 20, 1847; Battle of Churubusco, August 20, 1847; Battle of Molino del Rey, September 8, 1847; Battle of Chapultepec, September 13, 1847; Assault and Capture of the City of Mexico, September 13-14, 1847. He received a brevet to Lieutenant Colonel, September 8, 1847, for gallant and meritorious conduct in the Battle of Molino del Rey.

He served as Acting Inspector-General, January 27-October 24, 1848; on Frontier Duty at Ft. Niagara, NY, 1849-1850; on Recruiting Service, 1850-1852; at Benicia, CA, 1852-1853; Ft. Humboldt, CA, 1853-1856. He was promoted to Major, 4th Infantry, February 3, 1855 and was in command of the District of Southern Oregon and Northern California, March 7-July 13, 1856, being engaged against the Rogue River Indians, March 16-July 10, 1856, when hostilities terminated by the removal of the Tribes.

Buchanan was on Recruiting Service, 1857, as Superintendent of the Western Recruiting Service at Newport, KY, July 1, 1857-July 1, 1859; on Court Martial Duty, 1859-1860; on detached service at Washington, D. C., 1860-1861; and on Frontier Duty at Ft. Dalles, OR, 1861 and at Los Angeles, CA, 1861. He was promoted to Lieutenant Colonel, 4th Infantry, September 9, 1861.

In the Rebellion he served in the defenses of Washington, D. C., November 27, 1861-March 10, 1862. He was in the Virginia Peninsular Campaign (Army of the Potomac), March-July, 1862, in command of 4th Infantry, to May 24, 1862, and subsequently of a brigade of Regular Infantry in Sykes' Division of V Corps, being engaged in the Siege of Yorktown, April 5-May 4, 1862; Battle of Gaines's Mill, June 27, 1862; Battle of Glendale, June 30, 1862; and Battle of Malvern Hill, July 1, 1862. He was breveted to Colonel, June 27, 1862 for gallant and meritorious services at the Battle of Gaines's Mill, VA.

In the Northern Virginia Campaign, August-September, 1862, he was engaged in the Battle of Manassas, August 30, 1862. In the Maryland Campaign (Army of the Potomac), September-November, 1862, he was engaged in the Battle of Antietam, September 17, 1862; Skirmish at Potomac Run, September 18, 1862; and March to Falmouth, VA, October-November, 1862. He was commissioned a Brigadier-General, U. S. Volunteers, November 29, 1862-March 4, 1863.

In the Rappahannock Campaign (Army of the Potomac), December, 1862-January 27, 1863, he was engaged in the Battle of Fredericksburg, December 13, 1862. He was in command of Ft. Delaware, March 16-April 14, 1863; as Assistant Provost Marshal General, Chief Mustering and Disbursing Officer, and Superintendent of Volunteer Recruiting Service, for New Jersey, April 29, 1863- November 6, 1864. He was promoted to Colonel, 1st Infantry, February 8, 1864.

Buchanan served in command of 1st Infantry, at New Orleans, LA, December 6, 1864-August 27, 1865; was on sick leave of absence, August 26-December 1, 1865. He was breveted to Brigadier-General, U. S. Army, March 13, 1865, for gallant and meritorious services at the Battle of Malvern Hill, and breveted to Major-General, U. S. Army, March 13, 1865, for gallant and distinguished services at the Battles of Manassas and Fredericksburg.

After the Civil War, Buchanan served as a member of various commissions. During the troublesome period of Reconstruction he was in command of the District of Louisiana, January 2, 1868-January 8, 1869 and the Department of Louisiana, January 8-March 31, 1869. His role in this period was much heralded. However, despite this generally favorable view, at the instruction of the Republican Radicals Buchanan forced upon the state a Black governor and a carpetbag legislature.

Thereafter he was in command of his regiment, April, 1869-December 15, 1870, at Ft. Wayne, MI and later Ft. Porter, NY. He retired December 31, 1870. He died November 29, 1878 at Washington, D. C., aged 67 years, and was buried in Rock Creek Cemetery.