William A. Clark was born around 1839 in Connecticut to William and Mary Clark. His father was a farm laborer and carriage maker. The family lived in Fairfield County, Connecticut. He married Pheebe Webb, and their son Howard was born around 1866. By the early 1860s, they were living in Danbury, Connecticut.
He enlisted in the Union army on August 1, 1862, and he mustered in as a private in Company C of the 17th Connecticut Infantry on August 28. The regiment took part in the Battle of Chancellorsville and the Battle of Gettysburg. He was probably wounded in the summer of 1863, and he spent several months recovering at Carver Hospital in Washington, D.C. Eventually, he began working at the hospital. In November 1863, he wrote that he felt a "duty" not to "let my brother soldier die unheard." He added that he would "rather be at the front in line of Battle than lay here an sea men suffer it [is] no very desireable job...and the man that can stand it is a strong minded man."
In May 1864, Union officials transferred him to Company D of the 19th Veterans Reserve Corps. He remained in Washington, and by 1864, he was serving as a hospital nurse. He supported President Abraham Lincoln in the election of 1864. He mustered out on August 19, 1865.
He returned to Danbury after the war, and he earned a living as a hatter. By 1870, he owned $300 of personal property. He died in Danbury on February 20, 1880.