Lewis initially intended on enlisting in the military, but failed his physical exam because of a bad knee.

Lewis joined the United States Foreign Service in 1954 and served as consular officer at Naples.

He was Deputy Director of the Policy Planning staff from 1974 until 1975, when he became Assistant Secretary of State for International Organization Affairs.

[5] As ambassador, Lewis had a major role in negotiating the Camp David peace talks in 1978 that resulted in a historic treaty between Egypt and Israel.

The Camp David Accords were signed September 17, 1978, and within months, Menachem Begin and Anwar Sadat were jointly awarded the Nobel Peace Prize.

In 2004, Lewis was among 27 retired diplomats and military commanders who publicly stated that the George W. Bush Administration did not understand the world and was unable to handle "in either style or substance" the responsibilities of global leadership.

In 2009, he featured in a documentary directed by Harry Hunkele called Back Door Channels: The Price of Peace.