Washington is a borough in Warren County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 6,461, reflecting a decline of 251 from the 6,712 counted in the 2000 Census, which had in turn increased by 238 from the 6,474 counted in the 1990 Census. The borough is located in the eastern most region of the Lehigh Valley.Washington was incorporated as a borough by an act of the New Jersey Legislature on February 20, 1868, from portions of Washington Township. The borough was named for George Washington, one of more than ten communities statewide named for the first president. The Borough of Washington is surrounded by Washington Township, which is one of five municipalities in the state of New Jersey with the name "Washington Township".