Little is known of MacWillie's birthplace, date of birth, or his early life. MacWillie passed his bar exam and established a legal practice in Texas. He later moved to La Mesilla in what is now New Mexico and resumed his legal career.
Following the outbreak of the Civil War, MacWillie became the Attorney General of the Confederate-claimed Arizona Territory in the newly designated capital of La Mesilla. Through the shrewd political efforts of his powerful friend John R. Baylor, MacWillie was selected in early 1862 to replace Baylor's rival Granville Henderson Oury as the territory's representative to the permanent Congress. Despite Arizona and New Mexico being taken over by the Union Army later in 1862, MacWillie continued to represent the territory throughout the First Confederate Congress (March 11, 1862 – February 17, 1864). He then served in the Second Confederate Congress until the end of the war.
MacWillie's activities following the war are uncertain, as is his date and place of death or his burial location.
- Beers, Henry Putney, The Confederacy: A Guide to the Archives of the Government of the Confederate States of America. Washington, D.C.: United States National Archives and Records Administration, 1986.
- Current, Richard N., Encyclopedia of the Confederacy. New York: Simon & Schuster, 1993. ISBN 0132759918.
- Journal of the Congress of the Confederate States of America, 1861-1865. Washington, D.C.: United States War Department, Government Printing Office, 1905.
- History of New Mexico. Volume II, Los Angeles, Chicago, New York: Pacific States Publishing Co., 1907.
- In several sources, his first name is given as Malcolm. See Beers, p 24, as well as the official Journal of the Congress of the Confederate States of America, p. 80. The Official Records consistently uses the name Marcus.
- Alternatively, Macwillie. See Current, p. 984.
- History of New Mexico, Volume II, p. 565.
- Current, p. 984.
- Granville H. Oury biography
- Beers, p. 24.