Born in Spring Hill, Kansas on 23 December 1889, he was a 1912 graduate of the United States Military Academy at West Point, New York.

During World War I, he was aide-de-camp to Major General David C. Shanks, the New York Port of Embarkation commander at Hoboken, New Jersey, for which he was one of twelve army officers who received the Navy Cross.

He became involved in the vast construction program of arsenals, depots, airbases and coastal defenses as the United States rearmed prior to its entry into World War II.

In this role, he was responsible for planning and overseeing the execution of MacArthur's major operations, including the New Guinea, Philippines and Borneo campaigns.

After graduation, he was appointed to the United States Military Academy at West Point, New York by U.S. Representative Charles Frederick Scott.

He was promoted to captain on 15 May 1917, a few weeks after the American entry into World War I, and was acting regimental adjutant from 9 June to 1 September 1917.

[2] With the United States now involved in the war, Chamberlin became aide-de-camp to Major General David C. Shanks, the New York Port of Embarkation commander at Hoboken, New Jersey.

His citation read: The President of the United States takes pleasure in presenting the Army Distinguished Service Medal to Stephen J. Chamberlin, Major (Infantry), US Army, for exceptionally meritorious and distinguished services to the Government of the United States, in a duty of great responsibility during World War I.

On returning to the United States in July 1936, he became assistant professor of Military Science and Tactics for the high schools of Los Angeles.

[9] At this time, the United States was embarking on a military buildup in response to a worsening international situation, which culminated in the outbreak of World War II on 1 September 1939.

As far as wasting a few dollars was concerned, the construction effort cannot hold a candle to Lend-Lease, the Marshall Plan, or the Military Assistance Program.

Had it not been for the courageous performance of those in charge of the War Department in the emergency, we might well have been defeated, and how then would the expenditure of a few millions have loomed in the long-range picture?

[15] On 19 April, MacArthur formally established his General Headquarters (GHQ), and Chamberlin was appointed its Assistant Chief of Staff, G-3.

[17] Chamberlin was responsible for planning and overseeing the execution of MacArthur's major operations, including the New Guinea, Philippines and Borneo campaigns.

Considering the huge distances involved and the necessity for working in the humid heat of equatorial islands, the performance was stupendous.

Their decentralized mode of planning was entirely different from the top-down approach used by GHQ, and Chamberlin found this a source of frustration, as it was difficult to extract information from them.

[21] When MacArthur began looking for a new chief of staff to replace Lieutenant General Richard K. Sutherland in 1945, he considered but rejected giving the post to Chamberlin.

[9] In September 1946, he was one of five American major generals who was made an honorary Commander of Order of the British Empire in the Military Division for his work with US Army Forces in Australia and GHQ SWPA.

[9] In 1949, he was chairman of a general officer committee which researched the role of race in the Army, and produced a report favoring the continuation of segregation and the maintenance of a quota limiting the number of African-Americans who could serve in uniform.