William R. Carr was born around 1839 in Maine to John and Mary Carr. His father was a hatter who owned $150 of real estate by 1850. He grew up and attended school in Limerick, Maine. By the early 1860s, he was working as a bootmaker.
He enlisted in the Union army in May 1861. As he explained to his father, “I enlisted for three months at first, but if they want us longer why we shall go.” After briefly serving in one or two other units, he mustered in as a private in Company I of the 12th Massachusetts Infantry. The regiment took part in the Second Battle of Manassas, the Battle of Antietam, the Battle of Fredericksburg, the Battle of Chancellorsville, and the Battle of Gettysburg. He was promoted to sergeant in October 1862. He was severely wounded at Gettysburg on July 1, 1863, and surgeons amputated his left leg. He died near Gettysburg on July 14, 1863.