In 1964, Harris was elected a delegate to the Democratic National Convention from the District of Columbia. She worked in Lyndon Johnson's presidential campaign and seconded his nomination at the 1964 Democratic Convention. In October 1965, President Johnson appointed her Ambassador to Luxembourg, a role she served in until the end of the Johnson Administration. She was the first African American woman named as an American envoy. She said of her appointment, "I feel deeply proud and grateful this President chose me to knock down this barrier, but also a little sad about being the 'first Negro woman' because it implies we were not considered before." Additionally, Johnson named her as alternate delegate to the United Nations General Assembly for the years 1966-1968.