Father William Harnett was born in Philadelphia, Pennsylvania, in 1822. He was the first American-born to enter the Augustinian Order in the Province of Saint Thomas of Villanova. He entered the novitiate and continued his studies in Italy, where at Genazzano his Novice Master was Blessed Stephen Bellesini. He returned to the United States in 1843 and taught Latin and Greek at Villanova College. He was ordained to the priesthood in 1844 by Bishop Francis Patrick Kendrick of Philadelphia.

Father Harnett was President of Villanova College from 1847 to 1848, 1850 to 1851 and 1855 to 1857. He also served at Saint Augustine's in Philadelphia, Pennsylvania, and at Saint Mary's, in Lawrence, Massachusetts. He died in Lawrence, Massachusetts on 28 March 1875, at the age of 55. He is buried at Saint Mary's Cemetery in Lawrence, Massachusetts.