Carnahan served in the House for another six consecutive terms, but failed to win the Democratic Party's nomination for his own seat in 1960.

He helped write such major legislation as the GI Bill, the Marshall Plan, the Area Development Act, and a revision of the Social Security statutes, was a delegate to the 12th General Assembly of the United Nations in 1957, and served as Congressional Advisor to the U. S. Delegation to the Second International Conference on Peaceful Uses of Atomic Energy in Geneva in 1958.

[4][5] In 1961, Carnahan was appointed by President John F. Kennedy as the first United States Ambassador to Sierra Leone.

He became Rotary District World Service Chairman, and inaugurated a program to aid in the education of children in Sierra Leone.

His daughter-in-law, Jean Carnahan, was appointed a United States senator in 2001 following the death of her husband Mel, just before his election to that body.

Carnahan Hall on the campus of Southeast Missouri State University, which houses the classes and offices of the Department of Political Science, was rededicated to him in 2004.