Example image of eyePlorer eyePlorer map for 'Roman Catholic Archdiocese of Baltimore': Catholic Church Particular Church United States Allegany County, Maryland Anne Arundel County, Maryland Baltimore Baltimore County, Maryland Carroll County, Maryland County Frederick County, Maryland Garrett County, Maryland Harford County, Maryland Howard County, Maryland Maryland Washington County, Maryland 1859 August 15 Holy See Precedence Primate (religion) Emmitsburg, Maryland Mount St. Mary's University St. Mary's Seminary and University American Revolutionary War Apostolic vicariate British colonization of the Americas Thirteen Colonies Vicar Apostolic of the London District Congress of the Confederation Treaty of Paris (1783) John Carroll (bishop) Pope Pius VI Catholic Church hierarchy Roman Catholicism in the United States 1784 Apostolic prefect November 26 1789 November 6 Metropolitan bishop 1820 1868 July 11 March 3 Roman Catholic Diocese of Richmond Roman Catholic Diocese of Wilmington Roman Catholic Diocese of Wheeling-Charleston Roman Catholic Diocese of Arlington 1939 July 22