Col. Addison A Hosmer was born in Oakdale, Mass. Feb.28, 1833,the son of General E.M. Hosmer. After a three year course in Union College, he studied and practiced law for a time in North Carolina. In 1860 he returned north to Boston, and at the breaking out of the Civil War was commissioned as first lieutenant of the 28th Massachusetts Volunteers,and acted as post quartermaster at Camp Camron. In 1862 he was transferred to the First Massachusetts Heavy Artillery and soon thereafter was promoted to captaincy. During the Maryland Campaign he was aid-de-camp on the staff of General Samuel D Sturges and participated in the second battle of Bull Run and the battles at South Mountain and Antietam.Later he was judge advocate of the court which tried Maj. McKinstry at St.Louis.This trial resulted in the exposure of large embezzlement.With his regiment he then took part in the defense of Washington when the Confederate Army planned attack of the capital.
In 1863 President Lincoln appointed him judge advocate with the rank of Major,and for years he labored in the Bureau of Military Justice.In 1864 he was acting Judge Advocate General during the absence of General Holt. He was Judge Advocate of the commission which convicted Maj. Wirz of the Confederate Army. He also assisted in the trial of the accomplices of the assassination of President Lincoln. He was a warm friend of the president.
Col. Hosmer commenced the practice of law in the District after the close of the war, being associated with N.P. Chipman who was later a delegate to Congress from the District. In 1877, due to ill health , he was compelled to abandon active practice. He was always much interested in military affairs and was one of the organizers, and a Vice President of the Army & Navy Club and a vice commander of the District Commandry of the Loyal Legion.