Sussex County is the northernmost county in the State of New Jersey. Its county seat is Newton. It is part of the New York City Metropolitan Area. As of the 2010 United States Census, the county had 149,265 residents, an increase of 5,099 (3.5%) from the 144,166 persons enumerated in the 2000 Census, retaining its position as the 17th-most populous county among the state's 21 counties. Based on 2010 Census data, Vernon Township was the county's largest in both population and area, with a population of 23,943 and covering an area of . The Bureau of Economic Analysis ranked the county as having the 131st-highest per capita income ($49,207) of all 3,113 counties in the United States (and the ninth-highest in New Jersey).
Because of its topography, Sussex County has remained relatively rural and forested area. The county is part of the Skylands Region, a term promoted by the New Jersey Commerce, Economic Growth, & Tourism Commission to encourage tourism. In the western half of the county, several state and federal parks have kept the large tracts of land undeveloped and in their natural state. The eastern half of the county has had more suburban development because of its proximity to more populated areas and commercial development zones.
Most of Sussex County's economy was based on agriculture (chiefly dairy farming) and the mining industry. With the decline of these industries in the 1960s, Sussex County was transformed into a bedroom community that absorbed population shifts from New Jersey's urban areas. Recent studies estimate that 60% of Sussex County residents work outside of the county, many seeking or maintaining employment in New York City or New Jersey's more suburban and urban areas.
The area of Sussex County and its surrounding region was occupied for approximately 8,000-10,000 years by succeeding cultures of indigenous peoples. At the time of European encounter, the Munsee Indians inhabited the region. The Munsee were a loosely organized division of the Lenape (or Lenni Lenape), a Native American people also called "Delaware Indians" after their historic territory along the Delaware River. The Lenape inhabited the mid-Atlantic coastal areas and inland along the Hudson and Delaware rivers. The Munsee spoke a very distinct dialect of the Lenape and inhabited a region bounded by the Hudson River, the head waters of the Delaware River and the Susquehanna River, and south to the Lehigh River and Conewago Creek. As a result of disruption following the French and Indian War (1756–1763) the American Revolutionary War (1775–1783) and later Indian removals from the eastern United States, the main Lenape groups now live in Ontario in Canada, and in Wisconsin and Oklahoma in the United States.
By the 1750s, residents of this area began to petition colonial authorities for a new county to be formed; they complained of the inconvenience of long travel to conduct business with the government and the courts. By this time, four large townships had been created in this sparsely populated Northwestern region: Walpack Township (before 1731), Greenwich Township (before 1738), Hardwick Township (1750) and Newtown Township (1751). On June 8, 1753, Sussex County was created from these four municipalities, which had been part of Morris County when Morris stretched over all of northwestern New Jersey. Sussex County at this time encompassed present-day Sussex and Warren Counties and its boundaries were drawn by the New York-New Jersey border to the north, the Delaware River to the west, and the Musconetcong River to the south and east. After several decades of debate over where to hold the sessions of the county's courts, the state legislature eventually voted to divide Sussex County in two, using a line drawn from the juncture of the Flat Brook and Delaware River in a southeasterly direction to the Musconetcong River running through the Yellow Frame Presbyterian Church in present-day Fredon Township (then part of Hardwick). On November 20, 1824, Warren County was created from the southern territory of the Sussex County.
The Highlands Region of Northwestern New Jersey has proven to possess rich deposits of iron ore. In the mid 18th century, several entrepreneurial colonists began mining iron in area around Andover, Hamburg, and Franklin present-day Sussex County and establishing forges and furnaces to create pig iron and bar iron. During the American Revolution, the Quartermaster Department of the Continental Army complained to Congress of difficulties in acquiring iron to support the war effort and the Congress ordered two colonels, Benjamin Flower and Thomas Maybury to take possession of the iron works at Andover in order to equip General Washington's army. According to local tradition, Andover Forge forged the The Great Chain used at West Point to keep British naval vessels from coming up the Hudson River during the Revolution, but other sources say the chains were forged in Orange County, New York. During the middle of the 19th century, under the management of Cooper and Hewitt, the Andover mine produced 50,000 tons of iron ore each year. The firm manufactured railroad rails and the country's first structural steel, which and led to the building of railroads and commercial development in the county. Iron from the Andover mines was fashioned into cable wire for the bridge built at Niagara Falls and for the beams used to rebuild Princeton University's Nassau Hall in Princeton, New Jersey after a fire undermined the structure in 1855. During the American Civil War, Andover iron found its way into rifle barrels and cannonballs just as it had during the Revolution years before.
As deposits were depleted, the iron mining industry began to diminish by the mid-19th century. During the late 19th century, prolific American inventor Thomas Edison began to explore the commercial opportunities of processing poor-quality low-grade iron ore to combat the growing scarcity of iron deposits in the United States. He began to purchase mining companies in Sussex County in the 1880s and consolidating their assets. He developed a process of crushing and milling iron-bearing minerals and separating iron ore from the material through large electromagnets, and built one of the world's largest ore-crushing mills near Ogdensburg. Completed in 1889, the factory contained three giant electromagnets and was intended to process up to 1200 tons of iron ore every day. However, technical difficulties repeatedly thwarted production. However, in the 1890s, richer soft-grade iron ore deposits located in Minnesota's Iron Range rendered Edison's Ogdensburg operation unprofitable and he closed the works in 1900. Edison adapted the process and machinery for the cement industry and invested in producing Portland Cement in other locations.