Allen was born on the 22nd of December, 1876, in Evansville, Indiana, and while there is not much recorded information on his early or later life, it is known that he joined the US Navy in 1899. When it was time to be deployed in support of the Mexican Campaign, Allen was a Lieutenant Commander (O-4) and his actions earned him the Medal of Honor. The citation reads:

For distinguished conduct in battle, engagements of Vera Cruz, 21 and 22 April 1914. In command of the 1st Seaman Regiment, Lt. Cmdr. Buchanan was in both days' fighting and almost continually under fire from soon after landing, about noon of the 21st, until we were in possession of the city, about noon of the 22d. His duties required him to be at points of great danger in directing his officers and men, and he exhibited conspicuous courage, coolness, and skill in his conduct of the fighting. Upon his courage and skill depended, in great measure, success or Failure. His responsibilities were great, and he met them in a manner worthy of commendation.

Allen received the Medal of Honor on the 4th of December, 1915 and remained in the Navy, going on to serve in World War 1. His actions while commanding the USS Downes, which escorted convoys in waters with enemy mines and submarines, earned him the Navy Cross. Allen was married to Mary in 1903 and they had one child, Charles, who would also join the US Navy, receive the Navy Cross, and retire as a Rear Admiral. Allen Buchanan retired from the Navy in 1932 after thirty-three years at the rank of Captain and he died on the 12th of January, 1940 at the age of 63. He is buried in Arlington National Cemetery with his wife and son: Section 9, Grave 5845.