[citation needed] At the outbreak of the Civil War, Birney entered the Union Army on May 22, 1861, as a captain in the 1st New Jersey Infantry Regiment.

Promoted to major of the 4th New Jersey Infantry Regiment, he participated in the battles of Second Bull Run, Chantilly, Fredericksburg, and, as a colonel, Chancellorsville.

[citation needed] Birney's brigade was transferred to Virginia and joined other black regiments to form the Third Division of the X Corps under the command of Maj. Gen. Benjamin F. Butler.

[4] Birney resided in Florida for several years after the war before moving north in 1874 to establish a law practice in Washington, D.C.[5] He served as U.S. Attorney for the District of Columbia and as a school board trustee until 1886.

He wrote profusely on the subjects of religion and history and authored a biography of his father, James G. Birney and His Times; the Genesis of the Republican Party, in 1890.

[6][7] Birney died at his home in Forest Glen, Maryland, on August 14, 1907, and was buried in the Oak Hill Cemetery in Georgetown (Washington, D.C.).