During World War II, as part of a special services company entertaining troops in the South Pacific, Paar was a clever, wisecracking master of ceremonies. More than once, his pointed jibes at officers nearly got him into trouble. Paar became renowned among servicemen, who thought he was even better than professional comedians.
Jack Paar came to the attention of RKO Radio Pictures in Hollywood, which hired him to emcee Variety Time (1948), a compilation of vaudeville sketches. Paar later recalled that RKO didn't know what to do with him. His producers, trying to decide what kind of screen characters he could play, compared Paar with other RKO stars. Finally, Paar said, one of the executives had an inspiration, and figured out who Jack Paar really was: "Kay Kyser, with warmth." Paar projected a pleasant personality on film, and RKO called him back to emcee another filmed vaudeville show, Footlight Varieties (1951). Paar was featured in a few films, including a role opposite Marilyn Monroe in Love Nest (1951).
Like fellow humorists Steve Allen and Henry Morgan, Jack Paar dabbled in motion pictures but was much more comfortable behind a studio microphone, broadcasting. Paar found loyal listeners nationally as the 1950-51 host of radio's The $64 Question on NBC. He appeared as a standup comic on The Ed Sullivan Show and hosted two TV game shows, Up To Paar (1952) and Bank On The Stars (1953), before hosting The Morning Show (1954) on CBS. In 1956 he hosted The Jack Paar Show on the ABC Radio network.
The Tonight focus was always on compelling conversation and Paar's guests tended to be literate raconteurs such as Peter Ustinov rather than actors selling their current films, while Paar himself was a superb storyteller. Further, Paar surrounded himself with a memorable group of regulars and semi-regulars, including Cliff Arquette (as the homespun "Charlie Weaver"), author-illustrator Alexander King, Tedi Thurman (NBC's sultry "Miss Monitor") and comedy actresses Peggy Cass and Dody Goodman. In 1959, Paar's gagwriter Jack Douglas became a bestselling author (My Brother Was an Only Child, A Funny Thing Happened to Me on the Way to the Grave: An Autobiography) after his regular appearances with Paar. Douglas's pretty Japanese wife Reiko often appeared, as did Hungarian sexpot Zsa Zsa Gabor, French comedienne Genevieve and several Brits as well; Paar enjoyed conversing with foreigners and knew their accents would spice up the proceedings.
During this time, Paar also made occasional appearances on the television game shows Password and What's My Line? On episode 215 of the latter, Paar filled in as guest panelist for Steve Allen, his predecessor at The Tonight Show.
The decision to censor the joke so angered Paar that the next night, February 11, he announced on the air that he was leaving the show, saying "I've made a decision about what I'm going to do. I'm leaving The Tonight Show. There must be a better way to make a living than this, a way of entertaining people without being constantly involved in some form of controversy. I love NBC [...] But they let me down."After finishing this monologue, Paar abruptly walked offstage, leaving his flustered announcer Hugh Downs to finish the show for him.
Less than a month later, Paar was convinced to return; on March 7 he opened his monologue with the now-famous line, "As I was saying before I was interrupted...I believe the last thing I said was 'There must be a better way to make a living than this.' Well, I've looked...and there isn't." He then went on to explain his departure with typical frankness: "Leaving the show was a childish and perhaps emotional thing. I have been guilty of such action in the past and will perhaps be again. I'm totally unable to hide what I feel. It is not an asset in show business, but I shall do the best I can to amuse and entertain you and let other people speak freely, as I have in the past."
Paar then began hosting a prime-time Friday night show on NBC, entitled The Jack Paar Program. Popular belief holds that The Ed Sullivan Show introduced the Beatles to American television audiences; In fact, on January 3, 1964 the group made their prime time debut on Paar's hour in film clips Paar had leased from the BBC, with Paar gently making fun of the band (the Beatles first U.S. television appearance was in a feature story on The Huntley-Brinkley Report on November 18, 1963). Paar's show had a world view, debuting acts from around the globe and showing films from exotic locations; most of the films were made on travels made by guests such as Arthur Godfrey or Paar himself (e.g., several visits with Albert Schweitzer at his compound in Gabon, West Africa and Mary Martin at her home in the jungles of Brazil). During the first half of 1964, another running feud pitted Paar against the show immediately preceding his program, David Frost's satire series That Was The Week That Was. A typical exchange would have That Was the Week That Was "signing off" the NBC Television Network just before the Paar program, with Paar responding that the show immediately preceding his was Henry Morgan's Amateur Hour (Morgan was a frequent guest on the earlier show). The mock feud suddenly evaporated when NBC moved That Was the Week That Was to a Tuesday night time slot for the 1964-65 season.
Paar's prime time show aired for three years, including guests such as Brother Dave Gardner, Peter Ustinov, Lawrence of Arabia's brother, Richard Burton, Oscar Levant, Lowell Thomas, Cassius Clay reciting his poetry to piano accompaniment by Liberace, an occasionally inebriated Judy Garland, Jonathan Winters, Woody Allen, Bill Cosby (whose nickname for Paar was "The Boss"), Bette Davis, Robert Morley, Dick Gregory and many others. The final closing segment of the series, broadcast on June 25, 1965, featured him sitting alone on a stool, sharing a discussion that he had with his daughter Randy, who called Paar's departure a sabbatical. Noting the origins of the term, he said that his own field was, though not completely used up, "a little dry recently." Then he called to his German shepherd, who came to him from the seats of what was, for once, an empty studio, and walked out. Johnny Carson precisely copied this format of hosting a clip show from a stool for his own farewell episode of The Tonight Show in 1992.
In 1986, NBC aired a special featuring Paar, titled Jack Paar Comes Home; the following year, a second special Jack Paar Is Alive and Well was broadcast by the network. Both of these specials were largely made up of kinescoped clips from Paar's prime time program, to which he maintained the copyright. In the course of promoting the first special, Paar guested on Johnny Carson's version of Tonight for the first time on November 18, 1986. He appeared again to promote the next one on December 17, 1987.