Ellis saw his first combat action at the First Battle of Bull Run. He was then asked by Charles H. Van Wyck to help train the 56th New York Volunteer Infantry, a three-years' volunteer regiment. On September 5, 1862, Ellis became the first colonel of the newly raised 124th New York Infantry.
Ellis encountered his initial combat experience at the First Battle of Bull Run. He was subsequently requested by Charles H. Van Wyck to assist in training the 56th New York Volunteer Infantry, a three-years' volunteer regiment. On September 5, 1862, Ellis became the first colonel of the newly formed 124th New York Infantry.