In March 1864 Decatur Dorsey was released by his owner and enrolled in the 39th United States Colored Infantry (USCT) in Baltimore, Maryland. In May he was promoted to corporal and then by July, to sergeant.

Dorsey served as color bearer for the regiment at the battle of the Crater, July 30, 1864. During this action he planted the colors upon the Confederate works in advance of his regiment and when they were driven back he carried the colors to the Union works to rally the men. This was what was written upon the citation for the Medal of Honor he was awarded after this battle.

Dorsey was honorably discharged in December 1865 while in Wilmington, North Carolina. He married soon after that and died in 1891, from the effects of typhoid and rheumatism he had contracted in Wilmington, at the approximate age of fifty-one.