See Wikipedia for a full account of his Civil War service and later life. President as a Democrat but lost to Abraham Lincoln by a large margin.
The 2 cent Playing Cards revenue stamp on the back has a stamp cancel in black with the date September 1, 1864. That may be the approximate date that he sat for his photograph. By that time, McClellan was in the midst of his campaign for the Presidency.
This seems to be a rather rare image of Major General McClellan. The carte-de-visite is in very good condition with minimal wear and soiling.
The photograph exhibits good or better tonal range and shows very good detail. A nice portrait image of this popular Union general!