Sanger remained in the army after the war, and served in a variety of assignments as he rose through the ranks in the 1870s and 1880s, including command of several batteries of the 1st Artillery.

[4][5] He served until the end of the war, and received promotion to captain and major by brevet in recognition of his heroism in the Bermuda Hundred campaign and the first and second battles of Deep Bottom, Virginia.

[6] This civil disturbance became known as the Brooklyn Whiskey War of 1871, and Sanger commanded a battery in the army response that ended the riot.

[6] From 1872 to 1875, Sanger was professor of military science at Bowdoin College, and he was promoted to captain in February 1875.

[6] In March 1898, Sanger was appointed inspector general of United States Volunteers and promoted to lieutenant colonel.

[6] He was discharged from the volunteers in June 1899, and performed staff duty in the office of the Assistant Secretary of War.

[6] In August 1899, Sanger was appointed director of the U.S. census of Puerto Rico and Cuba, which was necessitated by the U.S. establishing governments there after defeating Spain.

[6] From 1901 to 1903, he served in the Philippines during the Philippine–American War, holding simultaneously the positions of chief of staff and inspector general for Adna Chaffee, the military governor.

[6] On January 20, 1904, he was promoted to major general in accordance with a law permitting Union veterans still on active duty to be advanced one grade before retiring.

He had authored articles for professional journals during his career, and after retirement, he was responsible for completing The Military Policy of the United States.

[9] This history of the army's doctrine and policy had been begun by Emory Upton, and it was edited for publication in 1904 by Sanger, William Dorrance Beach, and Charles Dudley Rhodes.

[9] He remained active in military and veterans' affairs, including serving on the Army of the Potomac committee that arranged the 1907 design and placement of Washington's Equestrian statue of George B.