Born in New York City on December 29, 1831, Badeau later became a clerk in New York City's Street Department.

In 1862, Badeau joined the Union Army during the American Civil War and was commissioned as a captain. Badeau joined the staff of Ulysses S. Grant as a brevet lieutenant colonel and military secretary in 1864.

During this time, Badeau and Grant became close friends. Badeau took part in the Wilderness and Appomattox campaigns and received promotion to brevet colonel. He remained on Grant's staff until 1869 and left the Army for disability; he held the permanent rank of captain and the brevet rank of brigadier general.

Among Badeau’s talents, he studied law and attained admission to the bar in 1855. He was a prolific author of both fiction and non-fiction, as well as authoring newspaper columns and magazine articles. Later in life, Badeau assisted Grant with the research, fact-checking, and editing of his autobiography.

Stan Southern will portray A. Badeau.