In April 1974, Nixon announced that he would appoint Cooper to be the US Ambassador to East Germany, but during the final negotiations between the countries for the US to establish an embassy in the country, Nixon resigned. His successor, Gerald Ford, officially appointed Cooper to the ambassadorship, and Cooper took leave from Covington & Burling to accept it. He arrived in East Germany in December 1974 and served as ambassador until October 1976. After returning to the US, he resumed his work at Covington & Burling. In his last act of public service, he again served as an alternate delegate to the UN General Assembly in 1981.