U.S. President Bill Clinton named Lewis the U.S. Ambassador to South Africa, a post in which he served from 1999 to 2001. He was sworn in by federal judge John Edwards Conway, a law-school classmate. Later, Lewis and his wife moved to Las Cruces, New Mexico, where he started a consultancy, Lewis & Associates. In 2006, he was named a senior fellow at New Mexico State University. The following year, he was named founding director of New Mexico State University's International Relations Institute.