After the Zwaanendael massacre, Sussex County passed through a sequence of European hands. The Dutch returned to settle the Hoernkills region around Cape Henlopen in 1663 with a Mennonite colony led by Pieter Cornelisz Plockhoy, only to have the English destroy that settlement the next year - so thoroughly, the English reports said, that not even a nail was left. The territory was claimed by Sweden, by the Dutch West India Company, and by both Lord Baltimore of Maryland and William Penn of Pennsylvania for the next century. Penn won the formal claim in 1682, organized the area as Sussex County (naming it after his ancestral home in England), and brought 200 colonists from England as settlers. Lord Baltimore did not accept the loss. Cresap's War, a series of armed clashes between Maryland and Pennsylvania settlers, raged through the 1730s. Only in 1760, when the Crown intervened and ordered the Calverts to honor a 1732 agreement, did the dispute end. The Mason-Dixon survey, run between 1763 and 1767, gave Sussex County its final western and southern boundaries.

Lewes served as the Sussex County seat for over a century, but the town sits on the Atlantic coast and was inconvenient for residents of the western part of the county. In 1791, the Delaware General Assembly approved moving the seat to a more central location. There was no existing town there - the commissioners simply bought 76 acres of farmland, hired surveyor Rhodes Shankland, and laid out a brand-new town centered on a 100-yard square. The plan extended outward in a circular pattern one mile in diameter. They named the new town for the chairman of the commission, Senate President George Mitchell - despite the obvious nickname potential, they called it Georgetown rather than Mitchellsville. The Circle, as the central square is called, is now listed on the National Register of Historic Places. Georgetown is a rare American town designed entirely from scratch in the late eighteenth century, with no preexisting settlement to negotiate around. The geometric plan is still readable on any aerial photograph.

Western Sussex County contains the Great Cypress Swamp - one of the northernmost natural stands of bald cypress trees in North America. The swamp covers roughly 50,000 acres straddling the Delaware-Maryland line, with the bald cypress trees concentrated in the wetter southern portion. Cypress is a southern species, and Sussex County's swamp sits at the edge of its range - the trees here are at the geographic limit of where they can survive winter cold. A massive fire in 1930 burned the surface of the swamp for eight months and changed its ecology permanently. Today the swamp supports threatened species, including the swamp pink lily and several bird species at the northern edge of their breeding range. Trap Pond State Park preserves part of the swamp and offers canoe and kayak access through the cypress knees. The Great Cypress Swamp is a piece of Louisiana that wandered north and stayed. The bald cypress trees, with their knobby root structures rising above the brown water, look out of place to most northeastern visitors - because they are.

Sussex County contains all of Delaware's Atlantic coast - from Cape Henlopen in the north through Rehoboth, Dewey, the Bethanies, and Fenwick Island. The beaches anchor the county's tourism economy and have transformed the eastern margin of the county from rural farmland to summer destination over the last seventy-five years. The inland bays - Rehoboth Bay, Indian River Bay, Little Assawoman Bay - sit between the barrier islands and the mainland. They form a complex of brackish water bodies that support oyster farming, recreational fishing, and the highest density of vacation homes per square mile in Delaware. The county's land area covers 48 percent of all of Delaware - the second-highest percentage of any U.S. county within its state. The county is the largest in Delaware by area. The land area also covers the major chicken-producing region of Delaware, where the poultry industry employs more people than the beach industry over the full year. The geography is two industries on one tract of land: tourism on the eastern margin, poultry across the rest.

Joe Biden's beach house, in the North Shores community between Rehoboth and Dewey Beach, has put Sussex County in the national political conversation more than the county's politics would predict. Sussex is the most conservative county in Delaware. The county has supported a Democrat for president only three times since 1944. Even in 2008 and 2012, when fellow Delawarean Biden was on the ticket as vice president, Sussex voted Republican. In 2020, when Biden won the presidency, he lost Sussex by 11 points. The contradictions matter: Biden won the county in six of his seven Senate races, including in 2008 when voters split their tickets to support him for Senate while rejecting him for vice president. The county council and the state legislators from Sussex are overwhelmingly Republican. The Delaware beaches have become the kind of place where the political class of the Northeast vacations in a county that did not vote for them. Biden's house, with its Secret Service detail, sits in a community that did not give him a majority. The neighbors complain about the motorcades. They sell each other holiday cards. The system, somehow, continues to work.