Duncan Stephen Walker was born on November 11, 1841, in Washington, D.C., to Robert and Maria Walker. His father was an attorney and Democratic politician who represented Mississippi in the United States Senate from 1835 until 1845. He grew up and attended school in Washington, D.C, before attending Georgetown University and Princeton University.
He remained loyal to the Union during the Civil War, and he received a commission as captain and assistant adjutant general in the Union army. He was promoted to major on July 27, 1864, and then to lieutenant colonel on November 7, 1864. He resigned on May 12, 1865, and he later received a brevet promotion to brigadier general.
He returned to Washington, D.C., after the war, and he earned a living as a lawyer. He married Mary Dodd, and they had at least four children: Alice, born around 1870; Edith, born around 1872; Robert, born around 1874; and Albert, born around 1875. By 1870, he owned $90,000 of real estate and $8,000 of personal property, and he employed at least two domestic servants, a cook, and a nurse.
Like his father, he supported the Democratic Party, and he served as secretary of the Democratic National Committee for several years. He moved to Hoboken, New Jersey, in the late 1800s, and he worked for the Hudson Observer newspaper. He died in Hoboken, New Jersey, on June 3, 1912, and he was buried at Arlington National Cemetery.