St. Mary's roots go back to 1741 when a small mission chapel was established in "Barnstown" under the guidance of the Jesuit missionaries who served St. John's Parish in Frederick, Maryland.

After fleeing France during the French Revolution, Father John Dubois became the pastor of St. John's Parish.

In 1807, Father Dubois recognized the growth of the mission and proposed to Bishop John Carroll of Baltimore

that a Catholic Church be built in upper Montgomery County. "Barnstown" was renamed "Barnesville" and became

the site for the new parish. The church immediately enjoyed community involvement withZachariah Knott donating 8 acres of land and another local parishioner donating nearby timber for its construction.

Fr. Dubois moved on to Emmitsburg where he established Mount St. Mary's and was also spiritual

director to St. Elizabeth Ann Seton. Later, as bishop of New York City, he ordained St. John Neumann to the priesthood.