Hewitt started working at the photo agency Acme Telepictures. Soon he received a lucrative offer at the CBS television network, which was seeking someone who had "picture experience" to help with production of televsion broadcast. Hewitt started at its news division, CBS News, in 1948 and served as producer-director of the network's evening-news broadcast with Douglas Edwards for fourteen years. He was also the first director of the landmark documentary news program See It Now, coproduced by host Edward R. Murrow and Fred W. Friendly that started in 1952.
Hewitt was the director of the 1960 U.S. Presidential-candidate debates—the first-ever televised presidential-candidate debates—between Richard Nixon and John F. Kennedy. He later became executive producer of the CBS Evening News with Walter Cronkite. He then launched the eight-time Emmy Award-winning show 60 Minutes.
Hewitt stepped aside as executive producer in 2004 at age 81. He is an eight-time Emmy Award winner. He is the author of Tell Me a Story: Fifty Years and 60 Minutes in Television, in which he chronicles his life as a newsman. He is also the author of the book Minute by Minute, a look at the history of 60 Minutes. On April 3, 2008, Hewitt was honored with Washington State University's Edward R. Murrow Award for Lifetime Achievement in Broadcast Journalism.