[2] In December 1863, he transferred to the 5th Wisconsin Infantry Regiment and served as a private in Company D. He served with the 5th Wisconsin Infantry in the Army of the Potomac in the eastern theater of the war and participated in the battles at the Wilderness, Spotsylvania Courthouse, North Anna, Cold Harbor, and the Siege of Petersburg.
In July 1864, he was commissioned as a 2nd lieutenant in the United States Marine Corps, but when it was determined that he was just 17 years old, he was forced to resign.
He was promoted to 1st lieutenant in July 1866 and served with the 4th U.S. Infantry Regiment through the end of 1870, working on railroad construction in the west.
[2] After leaving the Army, Sloan returned to studying law, this time at his father's office in Beaver Dam.
[4] After the 1895 legislative session, he was appointed district attorney of Douglas County to fill the unexpired term of C. J. Monson, who had died unexpectedly.
[2] Henry Sloan married Helen Lois Phinney of Green Lake, Wisconsin, on November 19, 1879.