Somerset County, Maryland was established on August 22, 1666 by a proclamation of Lord Baltimore through his Governor, Charles Calvert. At the time it was the entire southern Eastern Shore of Maryland south of the Nanticoke River.
The name comes from Lady Mary Somerset, the wife of Sir John Somerset. Lady Somerset was the sister of Lady Anne Arundell, who was the wife of Cæcilius Calvert, 2nd Lord Baltimore. They were daughters of Thomas Arundell, 1st Baron Arundell of Wardour. Anne Calvert, Baroness Baltimore, née Anne Arundell lent her name to Anne Arundel County.
In 1659/1660 Virginia passed a law that required Quakers in the colony to convert to Anglicanism or leave the colony. They decided to leave and move north to Maryland, who welcomed them. In November 1662 The Quakers settled a bit north of the southern boundary of Maryland on the southern bank of the Annemessex River which had been established at the mouth of the Pocomoke River and marked by a rock outcropping labelled as “Watkins’ Point”. This point would continue across the peninsula. A separate group of Anglican Virginian settlers were granted permission to make a second settlement further north along the Manokin River.
When established the Northern and Southern Borders of the county were in doubt. The Virginia Assembly declare that the Virginia-Maryland border was to be 30 miles north of the Pocomoke Sound, at the mouth of the Wicomico River. Both settlements were south of this point. Maryland authorities were outraged and appealed to the Virginia Governor, who agreed with Maryland’s claims. An agreement of the border between Maryland and Virginia Chesapeake Eastern Shores was agreed upon in June of 1668.
The Northern Border was in disagreement between the Maryland and the Colony of Pennsylvania (and Delaware) and wasn’t resolved until 1763 when surveyors Charles Mason and Jeremiah Dixon drew the definitive boundary between Delaware and Maryland. The Western border was the Chesapeake Bay with the Eastern border being the Atlantic Ocean.
Since then portions of the originally established Somerset County has been divided to create the Maryland counties of Worcester and Wicomico. The current county is 611 Square Miles with the land area as 327 Square Miles and 284 Square Miles of water. The 2010 census has the population at 26,470.