William W. Barnett was born in Pennsylvania on 15 September 1840 and raised in Armstrong Co. in western Pennsylvania by parents Alexander and Hannah Barnett. The 1860 census shows him living in the Borough of Freeport of that county as a 19 year-old student with his parents; brothers Henry B. (age 21), Robert A. (age 10), Hezekiah W. (age 9); and sister Emily W. (age 16). He enlisted as a private in Co. A, 8th Pennsylvania Reserve Volunteer Corps (PRVC), also known as the 37th Volunteers, on 15 May 1861, a few days after his brother Henry enlisted in Co. G, 9th PRVC. The 8th and the 9th saw duty in and around Washington D.C., before marching to Hunter's Mill in Fairfax Co. on 10 March 1862 and returning to Alexandria four days later. Barnett would see action in Virginia during June at Mechanicsville and Gaines Mill in the Seven Days Campaign and, later that summer, at the second battle of Bull Run. Before his regiment engaged at Antietam in mid-September 1862, Barnett fell ill and was left behind at Harewood Hospital in Northeast Washington on the farm of W. W. Corcoran. On 27 September, he was assigned to duty as a nurse at the hospital, and though he would become sick again before the end of 1862, he would finish the year serving at the hospital. He was discharged from service with the rank of private on a Surgeon's Certificate on 20 March 1863, some three months after his brother Henry was similarly discharged.

After, presumably, returning home to western Pennsylvania, Barnett responded to the local call to form the 5th Pennsylvania Heavy Artillery (also known as the 204th Regiment, Pennsylvania Volunteers) in August 1864. He was mustered in as a private, Battery M, on 2 September 1864. The regiment saw duty in the defense of Washington and along the Manassas Gap Railroad with engagements following at Salem and Rectortown in October 1864 before returning to Washington. Barnett was promoted to Sergeant and on 19 January 1865 was promoted to Second Lieutenant. He returned to Pittsburgh and mustered out with his battery on 30 June 1865.

Very little of Barnett's later life is known. An annotation added at a later date to Barnett's 1862 diary entry for 29 January reads, "Son of W. W. Barnett Wyllie Barnett was born January Friday 29 1875 - W. W. Barnett died Tuesday September 29 1876." A pension claim filed on 18 September 1890 by his widow cites the date of his death as Sept. 26 1876. As to the identity of his wife, nothing is known for certain, although the 1880 census does show a Rebecca Barnett living in the eastern Ohio county of Tuscarawas with her parents, Thomas and Sarah Laughead along with her five-year-old son, William Barnett.