Gwynne was born in New York City, a son of Frederick Walker Gwynne, a partner in the securities firm Gwynne Brothers, and his wife Dorothy Ficken. His paternal grandfather was an Episcopal priest born in Camus, near Strabane, County Tyrone, Northern Ireland, and his maternal grandfather was an immigrant from London, England. Gwynne attended the Groton School, and graduated fromHarvard University, where he was affiliated with Adams House, in 1951. Although Gwynne grew up in Tuxedo Park, New York, he spent most of his childhood in South Carolina, Florida, and Colorado because his father traveled extensively. At Harvard, he was a member of the Fly Club, sang with the a cappella group the Harvard Krokodiloes, was a cartoonist for the Harvard Lampoon(eventually becoming its president), and acted in the Hasty Pudding Theatricals shows.
During World War II, Gwynne served in the U.S. Navy. He later studied art under the G.I. Bill.
Gwynne joined the Brattle Theatre Repertory Company after graduation, then moved to New York City. To support himself, Gwynne worked as a copywriter for J. Walter Thompson, resigning in 1952 upon being cast in his first Broadway role, a gangster in a comedy called Mrs. McThing, which starred Helen Hayes.
Phil Silvers was impressed by Gwynne from his work in Mrs. McThing and sought him for his television show. As a result, in 1955, Gwynne made a memorable appearance on The Phil Silvers Show, in the episode "The Eating Contest" as the character Private Ed Honnergar, whose depressive eating binges are exploited by Sgt. Bilko (Phil Silvers), who seeks prize money by entering Honnergar in an eating contest. Gwynne's second appearance on The Phil Silvers Show (in the episode "Its For The Birds" in 1956 in which Bilko persuades bird expert Honnergar to go on The $64,000 Question) and many other shows led writer-producer Nat Hiken to cast him in the sitcom Car 54, Where Are You? as Patrolman Francis Muldoon, opposite Joe E. Ross. During the two-season run of the program he met longtime friend and later co-star, Al Lewis. Gwynne was 6 ft 5 (1.96 mt) in tall, an attribute that contributed to his being cast as Herman Munster, a goofy parody of Frankenstein's monster, in the sitcom The Munsters. For his role he had to wear 40 or 50 lbs of padding, makeup, and 4-inch elevator shoes. His face was painted a bright violet because it captured the most light on the black-and-white film. Gwynne was known for his sense of humor and retained fond recollections of Herman, saying in later life, "... I might as well tell you the truth. I love old Herman Munster. Much as I try not to, I can't stop liking that fellow." After his experience in The Munsters,however, he found himself typecast. In 1969, he was cast as Jonathan Brewster in a television production of Arsenic and Old Lace.
A talented vocalist, Gwynne sang in a Hallmark Hall of Fame made-for-television production, The Littlest Angel (1969), and went on to perform in a variety of roles on stage and screen. In 1974, he appeared in the role of Big Daddy Pollitt in the Broadway revival of Cat on a Hot Tin Roof with Elizabeth Ashley, Keir Dulleaand Kate Reid. In 1975 he played the Stage Manager in Our Town at the American Shakespeare Theatre in Stratford, Connecticut. He returned to Broadway in 1976 as Colonel J. C. Kinkaid in two parts of A Texas Trilogy. In 1984, he tried out for the part of Henry on the show Punky Brewster. He is said to have withdrawn from the audition in frustration when the auditioner identified him as Herman Munster rather than by his real name. The role of Henry subsequently went to George Gaynes. In 1987, Gwynne starred in a short-lived TV series Jake's M.O. where he played an investigative reporter.
Gwynne's performance as Jud Crandall in Pet Sematary was based on author Stephen King himself, who is also quite tall — only an inch shorter than the actor — and uses a similarly thick Maine dialect. Gwynne also had roles in the movies Simon, On the Waterfront, So Fine, Disorganized Crime, The Cotton Club, Captains Courageous, The Secret of My Success, Water, Ironweed, Fatal Attraction and The Boy Who Could Fly. Despite his misgiving about having been typecast, he also agreed to reprise the role of Herman Munster for the 1981 TV reunion movie The Munsters' Revenge. In his last film, Gwynne played Judge Chamberlain Haller in the 1992 film comedy My Cousin Vinny, in which he used a Southern accent, and his verbal sparring with Joe Pesci's character over how to pronounce the word "youths" was prominently featured in the film's trailer.
In addition to his acting career, Gwynne sang professionally, painted, and wrote and illustrated children's books, including It's Easy to See Why, A Chocolate Moose for Dinner, The King Who Rained, Best In Show, Pondlarker, The Battle of the Frogs and Mice, and A Little Pigeon Toad. Many of these efforts were based on children's frequent misperceptions of things they hear from adults, such as the "chocolate moose for dinner," which was illustrated as a large brown quadruped seated at the dinner table. The other books on this theme were "The King Who Rained," "A Little Pigeon Toad" (in which a child's mother thus describes her father), and "The Sixteen Hand Horse." Perhaps one of the reasons the books did not achieve wider popularity was the fact that their format was geared to a very young audience, but the concept itself was more appealing to older children and adults. He also lent his voice talents to commercials and radio shows such as CBS Radio Mystery Theater, and for some radio fans, he is known foremost for his contribution to CBSRMT's success. Later, he held a number of shows of his art work, the first in 1989.
In 1952, Gwynne married socialite Jean Reynard, a granddaughter of New York City mayor William Jay Gaynor. They had five children—three sons, Evan, Dylan, and Keiron, and two daughters, Madyn and Gaynor—before divorcing in 1980. Dylan died in a drowning accident as a child in 1963, and Keiron was born with developmental disabilities. In 1988, Gwynne married Deborah Flater.
Gwynne died of pancreatic cancer in Taneytown, Maryland, on July 2, 1993, at the age of 66, eight days before his 67th birthday. Gwynne was survived by his second wife, Deborah, and four children. He is interred at the Sandymount United Methodist Church graveyard in Finksburg, Maryland, in an unmarked grave.