The supporting cast includes Florence Rice, Kenny Baker, Margaret Dumont, and Eve Arden.

Jeff's friend and circus employee, Antonio 'Tony' Pirelli, summons attorney J. Cheever Loophole to investigate the situation.

Loophole discovers Carter's moll, Peerless Pauline (whose circus act consists of walking upside-down with suction cups on her shoes), is hiding the money, but she outwits him and he fails to retrieve it.

The audience is delighted with the circus; when the blustery Jardinet arrives, Loophole, who delayed the Frenchman's arrival on the SS Normandie by implicating him in a dope ring, disposes of the conductor and his orchestra by having Tony and Punchy cut the moorings on a floating bandstand as they play Wagner's prelude to act III of Lohengrin at the water's edge.

Comedy legend Buster Keaton's career had long been on the downside, and he was reduced to working for scale at MGM as a gag man.

As a gag, At the Circus screenwriter Irving Brecher masqueraded as Groucho in publicity stills alongside Harpo and Chico.

Groucho as J. Cheever Loophole was originally introduced in a key scene set in a courtroom which was filmed, but cut from the picture.

Groucho said he received many inquiries about this, and some viewers he happened to meet would ask him why the gorilla was noticeably smaller in the second half of the picture.

"[4] Variety called the film "broad, ribald fun in familiar pattern to early pictures of the Marx Bros."[5] Though Film Daily wrote, "The mad Marxmen have never been funnier, nor have they had a better story in which to cavort than 'At the Circus,'"[6] Harrison's Reports called it "about the worst Marx picture seen in years ... Children should enjoy it, but hardly any adults.