Warren County came into existence in 1825 when an act of the New Jersey Legislature, passed on Nov. 20, 1824, took effect and separated the area from Sussex County. The county was named in honor of Dr. Joseph Warren who, although he had no known association with the area, earned a heroic reputation throughout the original 13 states for his devotion to the revolutionary cause, which led to his death in the battle of Bunker Hill. During the Revolutionary War, a large majority of the 13,000 colonists who lived in Sussex (and what is now Warren) supported the patriotic cause. Sussex was among three New Jersey counties commended by the Provincial Congress in 1775 for "spirited exertions" in raising minutemen for the fight for independence.

The county originally had the townships of Greenwich, Hardwick, Independence, Knowlton, Mansfield, Oxford and Pahaquarry, from which the present 22 municipalities were carved. The Town of Belvidere was chosen as the county seat, a decision that was considerably influenced by the action of General Garrett D. Wall, who donated grounds for a county courthouse and public square in Belvidere.

The countyís freeholders ordered $3,500 raised toward construction of a brick courthouse with a jail and offices for the clerk and surrogate on the first floor and a courtroom (still in use today) on the second floor. The final accounting for the project was $9,942.24.

Transportation has played an important role in the development of the county since its beginning. Its earliest residents were the Lenni Lenape, who lived along the rivers and streams of the region. The first Europeans to settle in the county were the Dutch, who came to Pahaquarry Township and dug for copper around 1650. During this period, they constructed a road from Pahaquarry to Kingston, N.Y., over which they transported the proceeds of their mining ventures. This road, the first commercial highway built in the United States, is still in existence today. Known as "Old Mine Road," it traverses the Delaware Water Gap National Recreation Area, a major visitor attraction in the county.

Although it was the first area to be settled by European colonists, Pahaquarry Township is no more. Largely vacated by the federal government's disputed plan to build the Tocks Island Dam, much of Pahaquarry later became federal and state parkland. With only a handful of residents left, the Township merged with neighboring Hardwick Township on July 2, 1997, reducing the number of municipalities in New Jersey to 566. Accomplished through special legislation, the merger was the first one in the state in 45 years.

In the early 1830s, the Morris Canal opened in New Jersey, with its western terminus at Phillipsburg on the Delaware River in Warren County. Some 33 miles of the canalís 102-mile route to Jersey City were in Warren County, and the canal provided a thoroughfare for the county's farm products to reach metropolitan markets. Villages such as Port Warren, Port Colden, Port Murray and Rockport owe their names and their existence to their location along the canal. The Morris Canal also brought together the anthracite coal from Pennsylvania with the limestone and iron ore from New Jersey in the formation of heavy industry. Railroads quickly replaced the canal and sped the development of industry within the county.

At one time, Phillipsburg was the crossroads for five railroads, each with its own freight yard and maintenance facility in the town. Also, one heavy manufacturing plant employed up to 4,500 people and provided a great deal of machinery to the World War II effort.

At Oxford are the preserved ruins of one of colonial New Jersey's first iron furnaces, and it holds the distinction of operating longer than any other colonial furnace, from 1741-1884. In 1835, Oxford Furnace became the first U.S. smelting operation to use the hot blast process, while in 1859 Peter Cooper's furnace in Phillipsburg was one of the first in the U.S. to experiment with the Bessemer process. During the 1860s, Cooper Furnace was the most productive iron furnace in America.

Today, sections of the Morris Canal are being transformed into a public greenway across the county. Moreover, because of its transportation prominence, Phillipsburg has been tapped as the location for a planned New Jersey Transportation Heritage Center, a museum to highlight how New Jersey shaped transportation history and how transportation shaped the state. In addition to being a former railroad center and terminus of the Morris Canal , the town is closely linked with transportation history in other ways. A ferry across the Delaware River was established there in 1739, and the town served as an important port for Durham boats and lumber rafts that plied the Delaware. The first commercially successful diesel electric locomotives were developed in the town, and dirigible mooring masts were built there as well.

Transportation continues to play an important role in the county's evolution. Interstate highways I-78 and I-80 cross the county, opening the area to many new residents who commute daily to their jobs in metropolitan New Jersey. Meanwhile, commercial and industrial areas are being created along the highway corridors, as Warren County's proximity to New York, Philadelphia and New Jersey's most heavily populated areas, along with the quality of life the county offers, make it an attractive area for homes and businesses.