This is a list of free, online New Castle County Histories and related resources for New Castle County, Delaware.

Online New Castle County Histories include:

• Fort Casimir, the Starting Point in the History of New Castle, in the state of Delaware, its Location  and History, 1651-1671, by Alexander B. Cooper, 1905, at GoogleBooks
• New Castle on the Delaware, 1651-1936, by Delaware Federal Writers’ Project WPA, 1936, at HathiTrust
• Records of the court of New Castle on Delaware 1676-1681, Vol 1, published by the Colonial Society of Pennsylvania, 1904, at HathiTrust, Vol 2 is limited (search-only)
• Contributions to the Early History of Bryan McDonald and Family, settlers in 1689, on Red Clay Creek, Mill Creek Hundred (or Township), together with a few Biographical sketches and other statistics of general interest to their lineal descendants, by Frank V. McDonald, A.B., 1879, free at Google Books
• Sketch of early ecclesiastical affairs in New Castle, Delaware, by Thomas Holcomb, 1890, at Google Books
• History of the Presbytery of New Castle, From its Organization, March 13, 1717 to 1888 by Rev. J.L. Vallandigham, D.D., LL.D., 1889, at Internet Archive
• Sketch of Early Ecclesiastical Affairs in New Castle, Delaware, and History of Immanuel Church by Thomas Holcomb, 1890, at Internet Archive
• and see Online Delaware Histories, Biographies, and Related Material for possible entries related to New Castle County

New Castle County currently neighbors Kent County, Delaware; Kent and Cecil Counties, Maryland, Chester and Delaware Counties, Pennsylvania; and Gloucester and Salem Counties, New Jersey.