High Quality Content by WIKIPEDIA articles! Allenhurst is a borough in Monmouth County, New Jersey, United States. As of the 2010 United States Census, the borough`s population was 496, reflecting a decline of 222 (-30.9%) from the 718 counted in the 2000 Census, which had in turn declined by 41 (-5.4%) from the 759 counted in the 1990 Census.