In 1971, President Richard Nixon selected Porter to replace David K. E. Bruce as head of the U.S. delegation at the Paris Peace Talks. Before those talks were ended, in 1973, Nixon named Porter Under Secretary of State for Political Affairs, with Porter holding this office from February 2, 1973 until February 18, 1974. Nixon then named Porter United States Ambassador to Canada, with Porter filling this post from March 13, 1974 until December 16, 1975.