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,[8][9][10] reflecting an decline of 251 (-3.7%) from the 6,712 counted in the 2000 Census, which had in turn increased by 238 (+3.7%) from the 6,474 counted in the 1990 Census.[19] 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