An avid reader, Davis was a bright student in the public schools of the District of Columbia. His strong interest in the military and in American history manifested itself in his high school days. Davis was in his first year at Howard University when the United States declared war against Spain in April 1898. After completing his final examinations for the year, he joined the infantry.
Because he had been in the military training program at Howard, Davis was appointed temporary first lieutenant in the Eighth Volunteer Infantry in July 1898. His battalion moved south and was stationed at Key West, Fla. , when the war ended. Mustered out of the service in March 1899, Davis opted for a military career.
He enlisted as a private in Troop I of the Ninth Cavalry, one of four African-American units in the regular army. His regiment became part of the military force used to put down the insurrection in the Philippines. By the end of his first year, Davis had been promoted to sergeant major. When the army's officer corps was expanded in 1901, having been encouraged by several officers, Davis took the officer candidate examination. Placing third, he was awarded the permanent rank of second lieutenant of cavalry in February 1901.
Many considered him the protege of Colonel Charles Young, then the ranking African-American officer in the American army.
Prior to World War I, Davis' career was typical of that of the few African-American officers in the American military establishment. He served as a cavalry officer at various forts within the continental United States. He assisted in the training of African-American National Guardsmen and was an instructor in military science at African-American colleges, including Wilberforce University in Ohio and Tuskegee Institute in Alabama.
From 1909 until 1912, Davis served as the military attache to Liberia. Though suffering from recurring bouts of blackwater fever, he made a number of recommendations for the reorganization of the Liberian military and national constabulary. When Pancho Villa raided along the United States-Mexican border, Davis was assigned there, reaching the rank of major by the time the United States entered World War I. Davis saw no combat service during the war; his unit, the Ninth Cavalry, remained in the Philippines, decimated of experienced personnel to staff the growing number of African-American troops in the army.
After World War I, Davis was reassigned to the teaching of military science at African-American colleges. He also conducted parties of visitors to France's battlefields and American military cemeteries there, a service for which he received more than one commendation.
When he was promoted to colonel in 1930, he was the only African-American line officer in the United States Army. In 1938, Davis was appointed to the command of the 369th (N. Y. ) National Guard. During the 1940 presidential election campaign, the Roosevelt administration was under pressure from African-American voters because of continued segregation in the military. Davis was promoted to brigadier general in October 1940, becoming the first African-American to achieve the rank of general. (At that time, the only other regular African-American officer was his son. ) Davis was assigned command of the Fourth Cavalry Brigade.
In June 1942, approaching the retirement age of sixty-five, Davis was mustered out of the service. Because of the expansion of the military with the spread of the war in Europe, Davis was immediately reactivated and assigned to the Inspector General's Office, a move that also had the effect of removing him from an active field command.
During World War II, Davis was primarily involved in inspecting the conditions of African-American troops, investigating incidents involving possible racial conflict in the military, and making recommendations concerning the involvement of African-Americans in the army. Davis also made several trips to Europe, including England, Italy, and France, where he served as special adviser to theater commanders and to other important American military personnel.
In 1948, with Davis' retirement pending, Senator Clyde H. Hoey of North Carolina introduced a bill to give Davis the permanent rank of brigadier general. On July 20, 1948, his official retirement ceremony was held in the White House, presided over by President Harry Truman.
He was a member of the Advisory Committee on Negro Troop Policies (commonly called the McCloy Committee).
In October 1902, while stationed in Wyoming, Davis married Elnora Dickerson of Washington, D. C. ; they had three children. (Their son, Benjamin Oliver Davis, Jr. , became an army general. )
In 1916, Davis' wife died from complications following the birth of their last child, and in 1919 he married Sadie Overton.
Following his fifty years of active military service, Davis and his wife resided in northwest Washington.