Civil War Union Army Officer. Served as a Major in the pre-War New Jersey Militia Hudson Artillery battery (which was made up of mostly German immigrants). When the war broke out, he immediately offered his militia artillery for service in the Federal Army, but was initially turned down by Regular Army officers who distrusted militia units. After the Union failure at the Battle of First Bull Run proved that the War would not be short, his offer was accepted and his battery was mustered in on August 18, 1861 as Battery A, 1st New Jersey Light Artillery, with him as its Captain and commander. After some time in Winter camp the battery was attached to the First New Jersey Brigade in the Army of the Potomac's' VI Corps. He commanded his unit during the Peninsular Campaign (where it lost a gun at the Battle of Gaines Mill), and the Battles of Second Bull Run, Antietam and Fredericksburg. An illness took him out of action for the Chancellorsville and Gettysburg Campaigns (when his battery helped devastate Pickett's Charge on the Third Day of the Battle of Gettysburg, the artillery was led by second-in-command Lieutenant Augustine N. Parsons). Captain Hexamer returned to led his artillery in the Rappahannock Station and Mine Run Campaigns. When General Ulysses S. Grant opened up his 1864 Overland Campaign, Battery A was part of that thrust into Virginia. During the 8 days around the Battle of Cold Harbor, Hexamer's Battery shot over 800 rounds at the Confederates, and supported the disastrous Union charge on June 3. He continued to lead Battery A through the Petersburg Campaign (witnessing another Union disaster - the Battle of The Crater in July 1864) until his enlistment ran out, and he was mustered out of Federal service on August 18, 1864, having received no brevet promotions or rewards for 3 years of constant service and leadership.