Born in New York City, Hewitt was educated there and entered Dartmouth College when he was 15, graduating in 1934. His acting debut was in a school production at age 10.
Hewitt first appeared on the New York stage in The Taming of the Shrew in 1935, starring Alfred Lunt and Lynn Fontanne. He later toured with them in that play. in 1936-37, he appeared again with Lunt and Fontanne in productions of Amphitryon 38 and The Sea Gull. His obituary in The New York Times noted that he "scored his biggest successes on Broadway in William Saroyan's Love's Old Sweet Song and John Steinbeck's The Moon Is Down."
During World War II Hewitt served in the US Army's Armed Forces Radio Service from 1943-1946.
Among the movies Hewitt appeared in are A Private's Affair, That Touch of Mink, Days of Wine and Roses, How to Murder Your Wife, Sweet Charity, and The Barefoot Executive.
He never became a major star, but he did have a lengthy career which included smaller parts in several well-known programs, including Alfred Hitchcock Presents, Maverick, 77 Sunset Strip Daktari, Leave It to Beaver, The Donna Reed Show, Dr. Kildare, Bewitched, I Dream Of Jeannie, The Lucy Show, F Troop, The Wild Wild West, Ironside, The Bob Newhart Show and The Phil Silvers Show. He made four guest appearances on Perry Mason, three of which he portrayed the murderer: in 1961 he played Bruce Sheridan in "The Case of the Wintry Wife" and Dr. Marcus Tate in "The Case of the Brazen Bequest," and in 1965 he played the role of Curt Ordway in "The Case of the Fatal Fetish". Possibly his most prominent roles were Detective Brennan in My Favorite Martian and the district attorney in How to Murder Your Wife.
Hewitt became a member of Actors' Equity Association in 1934, gaining membership on its council in 1940. He served on the council until 1951 and was also on several committees during that span. An obituary noted, "He helped to establish the employment survey for performers in the American theater and frequently wrote letters and articles about actors' rights as professionals."
Hewitt died of cancer on November 7, 1986, at Memorial Sloan Kettering Cancer Center in New York City. He was survived by his mother and a brother.
Hewitt's papers were donated to Dartmouth College by his estate. The material housed at the Dartmouth College Library includes "programs, scripts, clippings, sides, reviews, correspondence, playbills, photographs and tapes." It occupies 22 boxes.