In April 1864 Dr Whistler was given a field assignment, becoming assistant surgeon in the 1st South Carolina Rifle Regiment or "Orr's Rifles". Arriving at his post in time for the battle of Spotsylvania Court House he made a favourable impression on his new comrades "[ordering] his servant to take his horse to the rear and out of danger, while he remained with the line of battle until it entered the Bloody Angle, and he was detained to look after such as had fallen in the charge." He took part in the battles of Jericho Ford, Riddle's Shop, Petersburg, Deep Bottom, Fussell's Mill, Ream's Station and Jones's Farm.