William Palmer Peirce was born around 1830 in Villenova, New York. He graduated from New York University in 1852 before moving to Illinois. He married Mary F. Rood on December 18, 1856, and the couple apparently had no children. He worked as a doctor in Lisbon, Illinois.
Peirce received a commission as a captain in Company D of the 36th Illinois Infantry, and he mustered in on August 20, 1861. According to his service records, he was 5 feet, 9½ inches tall, with black hair and black eyes. On September 3, 1862, he became an assistant surgeon in the 88th Illinois Infantry. He was promoted to surgeon on February 16, 1863, and he mustered out on June 9, 1865.
Peirce settled in Aux Sable, Illinois, after the war and resumed his work as a doctor. He supported the Republican Party, and he served in the Illinois legislature in the late 1860s. He was elected to the state senate in 1872. By 1870, he owned $2,000 of real estate and $18,200 of personal property, and they employed a Prussian domestic servant. His wife probably died in the 1870s, and he married a woman named Ella soon afterward. They had at least four children: William, born around 1879; James Garfield, born on September 12, 1881; Gurdon, born around 1884; and John, born around 1887.
By 1880, they were living in Grant, Illinois. He served as mayor of Hoopeston, Illinois, in the late 1880s. He applied for a federal pension in October 1891 and eventually secured one. He died in Hoopeston on February 20, 1911.