Ending Sanctuary Cities Act of 2021 This bill makes state or local governments that violate immigration laws ineligible for federal financial assistance. A governmental entity shall be ineligible if it (1) violates federal law by prohibiting its officials from communicating with the Department of Homeland Security (DHS) about the immigration status of any individual, (2) restricts compliance with DHS detainer requests, or (3) has any laws or policies that violate immigration laws. (A DHS detainer request is an order or request to a state or local government to temporarily hold or transport an alien for transfer into federal custody or notify DHS before releasing an alien from custody.) A government or law enforcement officer that complies with a DHS detainer request shall be acting under the color of federal authority, for the purposes of liability or immunity in a lawsuit filed by the detained individual. The bill makes it unlawful for a state or local government to discharge or discriminate against one of its law enforcement officers for complying with a DHS retainer request.