Source
http://behind.aotw.org/2007/06/26/witness-to-murder-henry-r-rathbone/
Henry had been born in Albany, New York to wealthy businessman and one-time Mayor (1838-41) Jared L. Rathbone. Sometime after Jared's death, his widow Pauline married State Supreme Court Justice Ira Harris, and Harris' daughter Clara (1834-1883) became Henry's step-sister.
Henry graduated from his step-father's alma mater Union College, Schenectady, in 1857, and the University Law School, Albany, then briefly practiced in Albany. He'd also held office in the State Militia before the War. In 1861 Judge Harris — friend to the new President — was himself elected US Senator from New York.
Clearly Henry was a privileged and well established young man.
At the outbreak of the Civil War he was given a commission as Captain in the 12th US Infantry, I'd guess partly because of his militia experience, but I should think mostly due to his political connections. He was on recruiting duty for the first year, raising a company for the Regiment in New York until March 1862. He joined his unit at Yorktown, on the Peninsula Campaign, presumably in command of a Company.
He was at Antietam as "acting field officer" (Major-like?) and in command of Company C of the Regiment's First Battalion (see Captain Blunt's report). The Battalion saw some light action on the west bank [map] of the Antietam across the Middle Bridge on September 17th.
Captain Rathbone remained with his unit to Fredericksburg in December, but from February 1863 to March 1864 he was away from the front on "special duty" at Washington City on the staff of the Military Governor. He then got an assignment to General Burnside's staff in the field as Aide-de-Camp and Commissary of Musters for the IX Army Corps. He saw action with that Corps at the Wilderness, Spottsylvania, Cold Harbor and on to the siege of Petersburg.
After the Crater debacle, and for the rest of the War, he was on headquarters duty in Washington in the Disbursing branch under the Provost Marshal. He was promoted Major of US Volunteers in March 1865.