William P. Sullivan was born in Ireland in 1824 and later resided in Franklin, Massachusetts. He had many laboring occupations, mainly as a cellar digger. He was married to Bridget Sullivan, age forty-nine, also from Ireland and they had seven children together. The children were between the ages of four and eighteen. At the age of thirty- eight he was enlisted into the civil war as a private. He was enlisted on July twenty-forth 1862. He served in company K, in the 33rd cavalry regiment of Massachusetts. He left for battle on August sixth in 1862. When he was Discharged from Company K, 33rd cavalry of Massachusetts on September twenty-ninth in 1864 for a total of two years served, during which he served for the union.
William's company fought in the siege of Port Hudson From June seventeenth- July ninth, 1863. Some other battle he participated in were Pleasant Hill, Monett's Ferry, Mansura, Yellow Bayou, Fishers Hill, and Cedar Creek. Williams part in the war was to be on guard and warning the other soldiers when the confederates were coming.