Charles Mortimer Rockefeller was born in Gallatin, New York, on September 18, 1844. He entered the Union Army in New York City as a private in Company E of the 7th New York Militia on April 26, 1861, after the start of the American Civil War. The 7th New York was held in reserve during the Battle of Bull Run and did not see action. Rockefeller was mustered out of service on August 2 of the same year.