Rev. William C. Dennis, a Virginia native, became the third pastor of Twelfth Baptist Church in 1876. His appointment came during the post-Reconstruction era, a time of significant challenges for Black communities in Boston and beyond.

Dennis was known for his dedication to his congregation and his efforts to foster resilience and community solidarity. Unfortunately, due to health issues, he resigned from his pastoral duties in 1880 after four years of service.

While the specifics of Dennis's life outside of his role at Twelfth Baptist Church remain scarce, his leadership is remembered as an important bridge between the church’s early abolitionist legacy and its continued commitment to community advocacy and spiritual guidance.