Civil War Medal of Honor Recipient. He enlisted in the 99th Pennsylvania Volunteer Infantry on December 14, 1861, and was mustered in as a Private in Company G. He would become an officer during his service in the field with the 99th Pennsylvania, and would render such distinguished and brave service at Boydton Plank Road during the 1864 Siege of Petersburg that he was awarded the CMOH. His citation for that award reads "Checked the rout and rallied the troops of his command in the face of a terrible fire of musketry; was severely wounded". Only months after his enlistment, his rise in rank began on April 1, 1862, when he was promoted to Corporal. One month later, on May 1, 1862, he was promoted to Sergeant, and participated with his unit in the Spring 1862 Peninsular Campaign at that rank. One of Company G's 2nd Lieutenants resigned at the end of May 1862, and Sergeant Bonnaffon was commissioned as a 2nd Lieutenant on August 1, 1862 to replace him. He then served in the field at that rank for the next two years, as his regiment fought in the battles of 2nd Bull Run, Fredericksburg, Chancellorsville, Gettysburg, Mine Run, The Wilderness, Spotsylvania, Cold Harbor and the campaign against Petersburg, Virginia during that time. On September 24, 1864 Sylvester Bonnaffon received his fourth promotion of the war, as he was advanced to 1st Lieutenant to fill a command vacancy that existed since 1st Lieutenant Isador Hirsh was killed on June 17, 1864. He was serving on the staff of 1st Brigade, 3rd Division, II Corps commander Brigadier General Regis de Trobriand at the Boydton Plank Road battle where he earned the Medal of Honor, and was specifically praised in General de Trobriand's official after-action report. On March 13, 1865 he was brevetted Major, US Volunteers for "gallant and meritorious services at the battle of Boydton Plank Road", and Lieutenant Colonel, US Volunteers for "gallant and meritorious services during the war". After his regiment saw the war though to its conclusion after the March-April 1865 battles of Five Forks, Sailor's Creek, and Appomattox, he was promoted for a fifth and last time on April 20, 1865, becoming Captain and commander of Company G. After the 99th Pennsylvania participated in the Grand Review Parade in Washington, DC on May 23, 1865, Captain Bonnaffon was honorably mustered out on July 1, 1865. In 1867 he was offered a commission of 2nd Lieutenant in the 31st United States Regular Infantry, but declined it. His post-war career saw him serve as cashier of customs for the port of Philadelphia, Pennsylvania, and as Colonel of the 20th Pennsylvania National Guard regiment. His Medal was awarded to him on September 29, 1893, nearly twenty eight years after his bravery.