St. John O'Sullivan (pronounced "sin-jin") (1874 –1933)[1] was a Catholic priest who personally undertook the restoration of the old Mission San Juan Capistrano in California.

He enrolled in Saint Bernard's Seminary in Rochester, New York, to do his theological studies, graduating in 1904, when he was ordained by the Bishop of Louisville, William George McCloskey.

[3] The pastor suggested that O'Sullivan might find the abandoned Mission San Juan Capistrano in California might provide him a place for him to exercise his ministry in a manner compatible with his health.

[2] In 1918 the Mission was given parochial status by the Bishop of Monterey-Los Angeles, John Joseph Cantwell, with O'Sullivan named as its first pastor in modern times.

[4] The O'Neill Museum was created at the Mission and serves headquarters of the San Juan Capistrano Historical Society and the repository of all its archives.