I pay income tax in a foreign country. Do I still have to file a U.S. income tax return even though I do not live in the United States?

You have to file a U.S. income tax return while working and living abroad unless you abandon your green card holder status by filing Form I-407, with the U.S. Citizen & Immigration Service, or you renounce your U.S. citizenship under certain circumstances described in the expatriation tax provisions.