Allan was born on November 12, 1837, at Winchester Gardens, near Winchester, one of two children, both sons, of Thomas Allan and Jane Dowdell George Allan. After being educated in a local private school, he taught in Jefferson County and in Winchester to earn enough money to enroll in the University of Virginia in 1857. Allan excelled at debate and graduated with honors in 1860 with an MA in applied arithmetic. He moved to Loudoun County where he was assistant to the principal of Bloomfield Academy when the Civil War began.
Allan enlisted in the Confederate army andas a clerk in the quartermaster department under Stonewall Jackson. In 1862, sponsored by University of Virginia classmate , Allan took the ordnance officer examination. He passed with the highest score and on December 27, 1862, became a of artillery. On January 19, 1863, he was appointed to Jackson’s staff as chief of ordnance of the Second Corps, Army of Northern Virginia. He served throughout the war, advancing to the rank of major on April 25, 1863, and lieutenant colonel on March 28, 1864. He was assigned to Jubal Early’s command on March 1, 1865.
After the war Allan took a job as cashier of the National Valley Bank in Staunton. In 1866 he accepted an invitation from Robert E. Lee, then president of Washington College in Lexington, to join the faculty as professor of applied mathematics. For almost eight years he taught there and published three books on applied mechanics between 1873 and 1875, all of which were reprinted shortly after his death.
While at Washington College, Allan wrote the first of many articles and books on the Civil War. He collaborated with Jedediah Hotchkiss on The Battle-fields of Virginia: Chancellorsville (1867) and also wrote a long unsigned article on the Battle of Gettysburg that appeared in the Southern Review in 1869. The piece on Gettysburg benefited from his interviews with Lee and was the first of a great many articles Allan wrote, so many that altogether they probably contained more words than his books. He became a popular figure on the lecture circuit and at commemorative ceremonies, and he published articles and speeches in Century, the Nation, and the Magazine of American History, but most of his articles, reviews, and commemorative pieces appeared in the Southern Historical Society Papers. His 1866 memoir of his own field ordnance service has been pronounced “priceless.”
Allan’s published work on the causes, conduct, and significance of the Civil War not only placed a substantial body of reliable information on the record, but also helped establish the literary genre of the Lost Cause. Allan’s most enduring and useful volumes on the Civil War were the History of the Campaign of Gen. T. J. (Stonewall) Jackson in the Shenandoah Valley of Virginia (1880), which was reprinted several times with variant titles and reissued in 1974, and The Army of Northern Virginia in 1862 (1892), published posthumously, the first part of an intended complete wartime history of Lee’s army.
On November 21, 1873, Allan was elected the first principal of McDonogh Institute, an endowed private school for poor boys at Owings Mills, near Baltimore, Maryland. He married Elizabeth Randolph Preston, daughter of a professor at the Virginia Military Institute, John Thomas Lewis Preston, on May 14, 1874, and they had two daughters and three sons. She was active for many years in the Presbyterian Church, in promoting Sunday schools, and in founding chapters of the Young Women’s Christian Association. She also became a successful author after her husband’s death, writing a novel published by the Congregational Sunday-School and Publishing Society, a biography of Margaret Junkin Preston, and her own posthumously published memoirs.
Under Allan’s leadership McDonogh Institute flourished. In 1885 he published a biographical tribute to the philanthropy of the institute’s founder, John McDonogh. In Maryland Allan remained active in organizations as diverse as the Society of the Army and Navy of the Confederate States, the Southern Historical Society, and the Military Historical Society of Massachusetts. From 1873 until his death he was a trustee of Washington and Lee University (as Washington College had become in 1871). He also served on the board of the Lee Memorial Association and prepared the historical sketch that the association used when raising money to build theat Washington and Lee. Allan died at his home at McDonogh School on September 17, 1889. Although his obituary in the Baltimore Sun stated that Allan was to be buried in Garrison Forest Cemetery, he was more likely interred in the churchyard at Saint Thomas’s Episcopal Church. His remains were then moved and reinterred in the Tagart Memorial Chapel at McDonogh School on or about December 4, 1898.
- The Battle-fields of Virginia: Chancellorsville (with Jedediah Hotchkiss; 1867)
- History of the Campaign of Gen. T. J. (Stonewall) Jackson in the Shenandoah Valley of Virginia (1880; reissued 1974)
- The Army of Northern Virginia in 1862 (1892)