Patrick Clark Hathaway was born around 1842 in Ohio to Ephraim and Sarah Hathaway. His father was a farmer who died in 1859. The family lived in Turtle Creek, Ohio, until the 1850s, when they moved to Washington, Ohio. By 1860, he was working as a clerk.
He enlisted in the Union army on August 5, 1862, and he mustered in as a private in Company A of the 35th Ohio Infantry. The regiment took part in the Battle of Perryville, the Battle of Stones River, the Battle of Chickamauga, the Battle of Missionary Ridge, and the Atlanta campaign. In October 1864, Union officials transferred him to Company C of the 18th Ohio Infantry. He mustered out on June 11, 1865.
He settled in Indianapolis, Indiana, after the war, and he died of consumption in Lebanon, Ohio, on June 27, 1872.