http://law.onecle.com/new-jersey/40-...es/41a-24.html

New Jersey Statutes - Title 40 Municipalities and Counties - 40:41A-24 Incorporation
The inhabitants of any county shall within the boundaries of that county be and remain a body corporate and politic, with perpetual succession
I was looking for the basis or fundamental law that gives authority to govern, cops seem to be always asking where do you reside. Look at the above cited section, it's the inhabitants that are the body politic (and corporate). The next section, tells you it's the registered voters who approve the direction of gov.

New Jersey Statutes - Title 40 Municipalities and Counties - 40:41A-25 Government of county after adoption of optional plan
40:41A-25. Government of county after adoption of optional plan
Upon adoption by the registered voters of any county of any of the optional forms of government set forth in this act, the county shall thereafter be governed by the plan adopted, by the provisions of this law applicable to all optional plans, and by all general laws, subject to the transitional provisions in article 7 of this act.
I haven't come across anything that says U.S. citizens are the prerequisite people to the organizing of county government.