[5] On his return to the United States, he began a medical practice in Boston, and became one of the district physicians for the city dispensary.

At the beginning of the Civil War, Green was commissioned assistant surgeon of the 1st Massachusetts Volunteer Regiment, and was the first medical officer mustered in for three years' service.

He was promoted surgeon of the 24th Massachusetts Regiment on September 2, 1861, where he remained until November 2, 1864, serving on the staffs of various cavalry officers.

In October 1863, he was sent to Florida, and was post surgeon at St. Augustine and Jacksonville; thence he was sent to Virginia and was with the army when Bermuda Hundred was taken.

Green organized Roanoke Cemetery in 1862, which was one of the first regular burial places for Union Army soldiers.

[1] During his time in office, the three members of the Boston Police Commission were removed and liquor license receipts increased by $22,000.