Created on May 3, 1850, from a part of Columbia County.

Applications to Take Oaths of Allegiance, 1940-1946. Minutes of the Board of County Commissioners, 1749-1824, 1933-1936, 1940-1970. Poor Directors and Poor Auditors Elections, 1877-1910. Minutes and Records of the Board of Relief, 1863-1865. Index to Delayed Birth Records, 1950-1968 Births, 1869-1906. Affidavits of Applicants for Marriage Licenses, 1885-1950. Minutes of the Board of Directors of the Poor and House of Employment, 1865-1892. Delayed Birth Records, 1941-1947 Births, 1870-1905.

Index to Marriage License Dockets, Male, 1885-1974. Birth Returns for Negroes and Mulattoes, 1803-1820. Will Abstracts and Vital Records of Mifflin and Mercer Counties, 1793-1880. Delayed Birth Records, 1941-1942 Births, 1879-1906. List of Retailers, Inns, and Taverns Licenses, and Elected Officials, 1830-1861.