Mill Basin is a residential neighborhood in southeastern Brooklyn, New York City. It is on a peninsula abutting Jamaica Bay and is bordered by Avenue U on the northwest and the Mill Basin/Mill Island Inlet on its remaining sides. Mill Basin is adjacent to the neighborhood of Bergen Beach to the northeast, Flatlands to the northwest, Marine Park to th…Mill Basin is a residential neighborhood in southeastern Brooklyn, New York City. It is on a peninsula abutting Jamaica Bay and is bordered by Avenue U on the northwest and the Mill Basin/Mill Island Inlet on its remaining sides. Mill Basin is adjacent to the neighborhood of Bergen Beach to the northeast, Flatlands to the northwest, Marine Park to the southwest, and Floyd Bennett Field and the former Barren Island to the southeast. Mill Basin also contains a subsection called Old Mill Basin, north of Avenue U.