Colonel Abraham Gilbert Mills was born on March 12, 1844 in New York, New York. He attended Union Hall Academy and graduated from Jamaica High School in Jamaica, New York. From his boyhood Mills played baseball, including a stint in right field with the Atlantic Base Ball Club of Jamaica. When the Civil War began, he enlisted as a private in the 165th New York Volunteers, also known as the Second Duryee Zouaves. He was commissioned a second lieutenant in 1864, and honorably discharged in 1865. His later rank of colonel was honorary. In 1869, Mills graduated from the Columbian—later George Washington—Law School in Washington, D.C. Mills was admitted to the D.C. bar, but never practiced law there. While in the capital, he organized the Olympic Base Ball Club and played alongside Nicholas E. Young, who would later succeed Mills as president of the National League.

In 1872, Mills married Mary Chester Steele and they had three daughters, Francis Steele Mills, Ellen Suydam Mills (Mather), and Mary Chase Mills (Lyall). His wife died in 1922, after which he lived with his daughter Mary and her family.

A.G. Mills became prominent in baseball circles in 1876 after proposing that the league ball clubs should have a reserve clause that protected the teams from losing players to better-financed clubs. He acted as the National League's counsel in the late 1870s, and succeeded William A. Hulbert as league president after Hulbert's death in 1882. Mills authored the first National Agreement, which included the first reserve clause that stipulated that players could not deal with other teams between April 1 and October 20. When team owners allowed players to return in 1884 after the players had ignored the reserve clause, Mills resigned as league president. He was succeeded by Nicholas E. Young, and was the last league president to not receive a salary.

Mills worked as a salesman in the elevator industry for many years. He started in the late 1870s with the Hale Elevator Company, which represented the Otis Elevator Company in Chicago. Otis later purchased Hale and from 1898 until Mills's death in 1929, he was the vice president of sales.

In 1905, sporting goods magnate Albert Spalding proposed the creation of a commission, known since then as the Spalding Commission or Mills Commission, to gather all possible information on the origins of baseball. A.G. Mills chaired the commission, which included Morgan G. Bulkeley, Arthur P. Gorman, Alfred J. Reach, George Wright, Nicholas E. Young, and James E. Sullivan as secretary. The commission issued its final decision in 1908 that baseball originated in the United States and that Abner Doubleday devised it in Cooperstown, New York, in 1839.

Other positions that Mills held included member and president of the New York Athletic Club; organizer of, and advisory counsel to, the American Olympic Association; commander of the Lafayette Post of the Department of New York, Grand Army of the Republic; secretary of the Survivors' Association of the Lafayette Post; officer of the Military Order of the Loyal Legion; officer of the French Legion of Honor; member and president of the Association for the Protection of the Adirondacks; and member of the Union League Club, American Museum of Natural History, and the Metropolitan Museum of Art. He also was active in the Amateur Athletic Union for many years.

A.G. Mills was eighty-five years old when he died of heart disease in Falmouth, Massachusetts, on August 26, 1929.