His father was a Scotsman named Edward Pierce who worked for the Hudson's Bay Company at Lax Kwʼalaams (a.k.a.
His "uncle by adoption" was the HBC employee and diarist Arthur Wellington Clah, and young William witnessed the famous event in which Clah intervened and saved the life of the Anglican lay missionary William Duncan, whose life was being threatened by Chief Ligeex of the Gispaxlo'ots, angry that church-bells were tolling on the day of his daughter's initiation into a secret society.
First informally and then formally after his ordination in 1886, Pierce worked to convert Natives and suppress indigenous customs (like the potlatch and secret societies) in B.C.
Pierce was missionary at Kispiox for fifteen years starting in 1895, and in 1910 was transferred to Port Essington, where he served until his retirement in 1933.
His book From Potlatch to Pulpit, published in 1933, contains memoirs of his life and conversion as well as substantial information about traditional Tsimshian customs, beliefs, and seasonal round.