After the war, Greene served on court-martial duty for a year and then returned to civil engineering in New York and Washington, D.C. From 1867 to 1871 he was the chief engineer commissioner of the Croton Aqueduct Department in New York.
At the age of 86, he inspected the entire 30-mile Croton Aqueduct structure on foot.
He served as president of the American Society of Civil Engineers from 1875 to 1877 and president of the New York Genealogical and Biographical Society.
He was appointed to West Point's Board of Visitors in 1881.

By 1892, Greene was the oldest surviving Union general and the oldest living graduate of West Point.
He petitioned the United States Congress for an engineer captain's pension that would be of help to his family after his death.
The best that Congress was willing to do was arranged by Congressman and Gettysburg veteran Daniel E. Sickles of New York, a first lieutenant's pension, based on the highest rank Greene had achieved in the regular army.
On August 18, 1894, Greene took the oath of office as a first lieutenant of artillery and became, at 93, the oldest lieutenant in the U.S. Army for 48 hours.
Veterans in the Military Order of the Loyal Legion of the United States (MOLLUS) declared that he was the oldest lieutenant in world history.
After the war, Greene served on court-martial duty for a year and then returned to civil engineering in New York and Washington, D.C. From 1867 to 1871 he was the chief engineer commissioner of the Croton Aqueduct Department in New York. At the age of 86, he inspected the entire 30-mile Croton Aqueduct structure on foot. He served as president of the American Society of Civil Engineers from 1875 to 1877 and president of the New York Genealogical and Biographical Society. He was appointed to West Point's Board of Visitors in 1881.
By 1892, Greene was the oldest surviving Union general and the oldest living graduate of West Point. He petitioned the United States Congress for an engineer captain's pension that would be of help to his family after his death. The best that Congress was willing to do was arranged by Congressman and Gettysburg veteran Daniel E. Sickles of New York, a first lieutenant's pension, based on the highest rank Greene had achieved in the regular army. On August 18, 1894, Greene took the oath of office as a first lieutenant of artillery and became, at 93, the oldest lieutenant in the U.S. Army for 48 hours. Veterans in the Military Order of the Loyal Legion of the United States (MOLLUS) declared that he was the oldest lieutenant in world history.
George S. Greene