Gordon was born in Brookline, Massachusetts. His stepfather, Bob Manning, was a crooner in the 1940s and ’50s of popular love songs, most known for his rendition of Hoagy Carmichael’s “The Nearness of You”. He began his professional life at age three; in his TV debut, he won second place on Ted Mack's Amateur Hour singing Johnnie Ray's "Cry". At six, Gordon recorded "Nuttin' for Christmas". He was the youngest performer ever to hit a pre-Hot 100 Billboard chart, when that song hit #6 in 1955. It sold over one million copies, and was awarded a gold disc. The next year he charted his second and final single, "Rock Around Mother Goose."
As a child star, Gordon also appeared on The Jackie Gleason Show, The Jack Benny Program, Richard Diamond, Private Detective, The Danny Thomas Show, Alfred Hitchcock Presents and Star Time with Benny Goodman. Gordon was cast as Humberto in an episode of the NBC sitcom Sally, starring Joan Caulfield, and as Chopper in ABC's Leave It to Beaver Episode 119: "Beaver's House Guest". Gordon guest starred on two CBS sitcoms, The Ann Sothern Show and Dennis the Menace, starring child actor Jay North.
In 1962 Barry played the part of the childhood version of the 'Patient' in "Pressure Point". When he was thirteen, Gordon began a career on Broadway, as Nick in Herb Gardner's A Thousand Clowns, a role that earned him a Tony Award nomination. He later reprised this role in the film version opposite Jason Robards and Martin Balsam in 1965. The film gave him "introducing" billing but he had actually been in several films prior, most notably his actual film debut in 1956's The Girl Can't Help It as a newspaper boy in which he uttered the classic line after seeing Jayne Mansfield, "If that's a girl, I don't know what my sister is."
Primarily as a character actor, Gordon became a familiar face in numerous feature films and television series. As a teen, Gordon starred alongside Sid Caesar and Vera Miles in the 1967 comedy-horror film, The Spirit is Willing. In the 1970s, Gordon appeared in the Barney Miller spin-off Fish, starring Abe Vigoda, and from 1973 - 1974 was a regular on The New Dick Van Dyke Show. In 1976, he appeared in an episode of the NBC situation comedy The Practice. In the last two seasons of the sitcom All in the Family, then known as Archie Bunker's Place, Gordon had the recurring role of Gary Rabinowitz, Archie's Jewish accountant. Gordon also had notable guest-starring roles on Star Trek: Deep Space Nine as Nava, a Ferengi businessman, and on Star Trek: Voyager as Ardon Broht, an alien publisher. More recently Gordon has appeared as the Rabbi in Larry David's Curb Your Enthusiasm.
Gordon is also a voice actor. His most notable voice roles were Donatello and Bebop in Teenage Mutant Ninja Turtles, and as Jake "Razor" Clawson in SWAT Kats: The Radical Squadron (1993). Gordon's voice was also featured in other animated series, such as Jabberjaw (as Cleveland "Clamhead" Rogers), Tarzan, Lord of the Jungle, The Kid Super Power Hour with Shazam! (as Captain Marvel Jr.), Meatballs & Spaghetti (as bassist Clyde), Pac-Man (as Inky), Mighty Orbots (as Robert Simmons), Pole Position, The Jetsons, The Smurfs, The Adventures of the American Rabbit (as the titular character), Superman, Snorks (as "Junior" Wetworth), Gravedale High (as Reggie Moonshroud), Space Cats, A Pup Named Scooby-Doo (as Englebert Newton), Darkwing Duck (as Dr. Fossil), Tom & Jerry Kids and its spin-off Droopy, Master Detective, Batman: The Animated Series (as the Penguin's henchman Sheldrake), Aaahh!!! Real Monsters, Timon & Pumbaa, Fantastic Max, and The Pirates of Dark Water. Gordon also provided the voice of "Quicky" the Nesquik Bunny in television commercials for Nestlé.
In his mid-thirties, Gordon returned to school; he graduated summa cum laude as a political science major from California State University, Los Angeles; and he went on to Loyola Law School, receiving his J.D. in 1991.
Gordon became the longest-serving president of the Screen Actors Guild, holding the office for seven years.
In 1998, Gordon was the Democratic Party nominee for the United States Congress from the Pasadena, California, area. He surprised political pundits of both parties by coming within three points of unseating the Republican Party incumbent, James Rogan.
In 2004, when the local Air America Radio affiliate in Los Angeles went off the air, for a then-unspecified period of time, Gordon started a live, call-in progressive political talk show on Pasadena's Public-access television cable TV channel 56. It continues to be cablecast and webcast live, with Adobe Flash video available on demand.
In 2005, Gordon hosted a weekly radio talk show heard on KRLA in Los Angeles, California.
In 2006 and early 2007, Gordon hosted Barry Gordon From Left Field, a weekly talk show broadcast throughout the 25th largest U.S. radio market—the San Bernardino/Riverside region of Southern California—on KCAA Radio, in Loma Linda, California. With live streams and podcast archives, the show was especially notable for featuring nationally known guests, including senators, congressmen, bestselling authors, and entertainment figures.
In 2008, Gordon debuted his daily Internet talk show, Left Talk, on BlogTalkRadio.
Since 2007, Gordon has taught courses in politics and the media at the California State University, Los Angeles.
In May 2009, Gordon played the Cocker Spaniel in the Webkinz Pet of the Month Music Video for May 2009.
Gordon reprised the role as the original Donatello in the second and fourth seasons of Nickelodeon's Teenage Mutant Ninja Turtles.