Wynn became a headliner in vaudeville in the early-1910s, and was a star of the Ziegfeld Follies starting in 1914. In The Ziegfeld follies, W. C. Fields allegedly caught him mugging for the audience under the table during his "Pool Room" routine and knocked him unconscious with his cue, but this may be apochryphal. He was best known as a comedian, billed as The Perfect Fool (and starring in a musical revue of that name on Broadway in 1921). Wynn also wrote, directed and produced many shows. He was famous for his silly costumes and props, and he always worked "clean," making his shows suitable for the entire family.

A Jewish-American who was born Isaiah Edwin Leopold in Philadelphia, Pennsylvania, he attended Central High School (Philadelphia) until age 15. He ran away from home in his teens, worked as a hat salesman and as a utility boy, and eventually adapted his middle name "Edwin" into his new stage name, "Ed Wynn", to save his family the embarrassment of having a low comedian as a relative.

Although many gag writers later provided material for Wynn's performances in radio, television and movies, it was his proud boast that every line he ever spoke during his early career as a stage performer was written by himself.

He hosted a popular radio show, The Fire Chief for most of the 1930s, heard in North America on Tuesday nights, sponsored by Texaco gasoline. Like many former vaudeville performers who turned to radio in the same decade, the stage-trained Wynn insisted on playing for a live studio audience, doing each program as an actual stage show, using visual bits to augment his written material, and in his case, wearing a colorful costume with a red fireman's helmet. He usually bounced his gags off announcer/straight man Graham McNamee; Wynn's customary opening, "Tonight, Graham, the show's gonna be different," became one of the most familiar tag-lines of its time. Sample joke: "Graham, my uncle just bought a new second-handed car... he calls it Baby! I don't know, it won't go anyplace without a rattle!"

Wynn was a radio superstar who reprised his radio character in two movies, Follow the Leader (1930) and The Chief (1933). Near the height of his radio fame he founded his own short-lived radio network, the Amalgamated Broadcasting System, which lasted only five weeks in 1933 and nearly destroyed the comedian, according to radio historian Elizabeth McLeod, who has written that the failed venture left Wynn deep in debt, divorced, and finally suffering a nervous breakdown.

Wynn was offered the title role in MGM's 1939 screen adaptation of The Wizard of Oz, but he turned down the role, as did his Ziegfeld contemporary W. C. Fields. The part finally went to Frank Morgan.

In the 1949-50, Ed Wynn hosted one of the first comedy-variety television shows, on CBS, and won an Emmy Award in 1949. Buster Keaton, Lucille Ball, and The Three Stooges all made guest appearances with Wynn. This was the first CBS variety television show to originate in Los Angeles, with programs filmed via kinescope for distribution in the Midwest and East. Wynn was also a rotating host of NBC's Four Star Revue from 1950 through 1952.

After the end of Wynn's third television series, The Ed Wynn Show (a short-lived situation comedy on NBC's 1958-59 schedule), his son, actor Keenan Wynn, encouraged him to make the career change rather than retire. The comedian reluctantly began a career as a dramatic actor in television and movies. Father and son appeared in three productions, the first of which was the 1956 Playhouse 90 broadcast of Rod Serling's play Requiem for a Heavyweight. Ed was terrified of straight acting and kept goofing his lines in rehearsal. When the producers wanted to fire him, star Jack Palance said he would quit if they fired Ed. (However, unbeknownst to Wynn, supporting player Ned Glass was his secret understudy in case something did happen before air time.) On live broadcast night, Wynn surprised everyone with his pitch-perfect performance, and his quick ad libs to cover his mistakes. A dramatization of what happened during the production was later staged as an April 1960 Westinghouse Desilu Playhouse episode, "The Man In the Funny Suit", starring both senior and junior Wynns, with key figures involved in the original production also portraying themselves. Ed and his son also worked together in the Jose Ferrer film The Great Man, with Ed again proving his unexpected skills in drama.

Requiem established Wynn as serious dramatic actor who could easily hold his own with the best. His role in The Diary of Anne Frank (1959) won him an Academy Award nomination for Best Supporting Actor.

Also in 1959, Wynn appeared on Serling's TV series The Twilight Zone in "One for the Angels". Serling, a longtime admirer, had written that episode especially for him, and Wynn later starred in the episode "Ninety Years Without Slumbering". For the rest of his life, Ed skillfully moved between comic and dramatic roles. He appeared in feature films and anthology television, endearing himself to new generations of fans.