Rev. William Sullivan Barnes was born on June 16, 1841, in Boston, Massachusetts.

He was a Unitarian minister who transitioned from the business world to theology by studying at Newton Theological Seminary. In 1864, he was ordained into the Baptist ministry in Melrose, Massachusetts. However, in 1868, due to his increasingly liberal theology and his advocacy for open communion, he left the Melrose church and withdrew from Baptist fellowship. He then accepted a position with the Church of the Messiah (Unitarian) in Montreal, Quebec, where he quickly gained recognition as an outstanding orator. Despite being hampered by shyness and physical frailty, including suffering from asthma—which sometimes necessitated finding a substitute for him on short notice—his personal magnetism drew people to his sermons. He embraced Darwinian evolution, various new scientific theories, and higher criticism of the Bible. His ministry focused heavily on culture and the visual arts, which contributed to his reputation. In 1909, he was awarded an honorary LL.D from McGill University.

However, Barnes was perceived as weak in promoting denominational interests, and the congregation diminished during his tenure.

The archives of the Unitarian Church of Montreal contain the minute books of the Church of the Messiah, a scrapbook with newspaper clippings about Barnes, transcriptions of several of his sermons, and manuscript histories of the congregation. Additionally, there are a few letters in the archives of the Unitarian Universalist Association in Boston, Massachusetts. Barnes also wrote the article on Unitarianism in Canada for "An Encyclopedia of the Country" in 1898.

In 1864, he married Mary Alice Turner (1843-1920). He died on April 2, 1912, in Montreal, Quebec.