The county is named after the strait between the Door Peninsula and Washington Island. The dangerous passage, which is now scattered with shipwrecks, was known to early French explorers and local Native Americans. Because of the natural hazards of the strait, where the waters of Green Bay meet the open body of Lake Michigan, they gave it the French appellation Porte des Morts Passage, which in English means the "Door to the Way to Death," or simply, "Death's Door."

The county has a total area of 2,370 sq mi. The county also has 298 miles of shoreline, more than almost any other county in the continental United States. Both Suffolk County, New York, on Long Island (980 miles) and Barnstable County, Massachusetts, which includes Cape Cod (550 miles), have more. This is one of the reasons that locals and tourists alike refer to the area as the Cape Cod of the Midwest. The county covers the majority of the Door Peninsula. With the completion of the Sturgeon Bay Shipping Canal in 1881, the northern half of the peninsula, in actuality, became an island. Limestone outcroppings of the Niagara Escarpment are visible on both shores of the peninsula, but are larger and more prominent on the Green Bay side as seen at the Bayshore Blufflands. Progressions of dunes have created much of the rest of the shoreline, especially on the easterly side. Flora along the shore provides clear evidence of plant succession. The middle of the peninsula is mostly flat or rolling cultivated land.

Door County has 12 lighthouses and hundreds of scenic farms.