The Mountebanks is a comic opera in two acts with music by Alfred Cellier and Ivan Caryll and a libretto by W. S. Gilbert. The story concerns a magic potion that causes the person to whom it is administered to become what he or she has pretended to be. It is similar to several "magic lozenge" plots that Gilbert had proposed to the composer Arthur Sullivan, but that Sullivan had rejected, earlier in their careers. To set his libretto to music, Gilbert turned to Cellier, who had previously been a musical director for Gilbert and Sullivan and had since become a successful composer. During the composition of the piece Cellier died, and the score was finished by the original production's musical director, Ivan Caryll, who became a successful composer of Edwardian Musical Comedy.

The opera was first produced at the Lyric Theatre, London, on 4 January 1892, for a run of 229 performances. It also toured extensively, had a short Broadway run, in 1893, American tours and Australian productions. The original cast included Geraldine Ulmar, Frank Wyatt, Lionel Brough, Eva Moore and Furneaux Cook. The American cast included Hayden Coffin and Lillian Russell. A professional recording of the work was released in 2018.

The story of the opera revolves around a magic potion that transforms those who drink it into whoever, or whatever, they pretend to be. The idea was clearly important to Gilbert, as he repeatedly urged his famous collaborator, Arthur Sullivan, to set this story, or a similar one, to music. For example, he had written a treatment of the opera in 1884, which Sullivan rejected, both because of the story's mechanical contrivance, and because they had already produced an opera concerning a magic potion, The Sorcerer (1877); Sullivan felt that the story lacked "human emotion". The idea of a magic potion that changes human behaviour has long been a common theme of literature and opera. The device allowed Gilbert to explore "how people behave when they are forced to live with the consequences of their own actions."

The Gilbert and Sullivan partnership and their Savoy operas dominated the London musical stage from the late 1870s to 1890. When that partnership temporarily disbanded, due to a quarrel over finances after the production of The Gondoliers, Gilbert sought another composer who would collaborate on the "magic lozenge" idea that Sullivan had repeatedly rejected. He eventually found a willing partner in Alfred Cellier, a logical choice for Gilbert. The two had collaborated once before (Topsyturveydom, 1874), and Cellier had been the musical director for Gilbert and Sullivan's early operas. Cellier had also achieved much success apart from Gilbert and Sullivan, particularly with his comic opera Dorothy (1886), a smash hit. It played for over 900 performances, considerably more than The Mikado, Gilbert and Sullivan's most successful piece. Dorothy set and held the record for longest-running piece of musical theatre in history until the turn of the century.

Gilbert and Cellier agreed to collaborate on The Mountebanks in July 1890, and Gilbert began fleshing out the libretto, but unlike his usual daily interactions with Sullivan during development of a libretto, he found Cellier to be far less responsive. He was annoyed when Cellier sailed for Australia in mid-December without having responded to Gilbert's repeated queries about potential conflicts between some plot changes that he had suggested and a recently composed opera of Cellier's with B. C. Stephenson, The Black Mask, including a Spanish setting involving guerillas during the Peninsular War. Gilbert then completed Act I assuming that there were no conflicts, but finally received a response from Cellier by early January, stating that the change in setting did indeed conflict with his earlier work; Gilbert replied that he was ending the collaboration, and that Horace Sedger, the manager and lessee of London's Lyric Theatre, where the piece was to be produced, agreed with this. In early February, Gilbert approached composer Arthur Thomas to set the libretto to music, and Thomas sketched out music to four musical numbers. For unknown reasons, possibly Thomas's poor health, he never set the opera; when Cellier returned to England in April 1891, he sought, through his and Gilbert's mutual friend, Edward Chappell, to mend fences with Gilbert, and, after some flattery, succeeded. Gilbert changed the setting to Sicily, and the guerillas became brigands; it turned out that The Black Mask was never produced.

Cellier suffered from tuberculosis for most of his adult life, but during the composition of The Mountebanks he deteriorated rapidly and died, at the age of 47, while the opera was still in rehearsals. All of the melodies and vocal lines in the opera were composed by Cellier, but the orchestration was incomplete when he died. The score was completed by the Lyric Theatre's musical director, Ivan Caryll, a successful composer who became one of the best-known composers of Edwardian Musical Comedy. Caryll composed the entr'acte, using the melody from Number 16, and he wrote or modified the orchestration for more than half a dozen of the songs. He chose the 4th movement of Cellier's 1878 orchestral piece, the Suite Symphonique, to use as the opera's overture. One song whose lyrics were printed in the libretto available on the first night were never set to music, and another was cut before the opening night. After Cellier's illness prevented him from finishing the score, Gilbert modified the libretto around the gaps, and the order of some of the music was changed.

The Mountebanks's initial run of 229 performances surpassed most of Gilbert's later works and even a few of his collaborations with Sullivan. Gilbert engaged his old friends John D'Auban, to choreograph the piece, and Percy Anderson, to design costumes. The initial run closed on 5 August 1892. Despite the opera's warm reception, Gilbert wrote on 7 January 1892, shortly after the premiere, "I had to make rough & ready alterations to supply gaps – musical gaps – caused by poor Cellier's inability to complete his work. It follows that Act 2 stands out as a very poor piece of dramatic construction ... this is the worst libretto I have written. Perhaps I am growing old."

The success of the London production led its producer, Sedger, to establish at least three touring companies, which visited major towns and cities in Britain for a year and a half, from March 1892 to mid-November 1893. Louie René played Ultrice on one tour in 1893. While playing in Manchester, one touring company found itself competing with a D'Oyly Carte Opera Company touring company at a nearby theatre. The strained relations between Carte and Gilbert after The Gondoliers did not prevent the two companies from playing a cricket match in May 1892. Relations between Gilbert and his new producer had also deteriorated, and the author unsuccessfully sued Sedger for cutting the size of the chorus in the London production without his approval. It was toured for a year in America by the Lillian Russell Opera Company, starring Lillian Russell and C. Hayden Coffin, including a run of a month and a half at the Garden Theatre on Broadway, opening on 11 January 1893. It was also produced in Australia and New Zealand by the J. C. Williamson company until 1900.