A shiny stainless steel sculpture created by Jeff Koons in 1986, inspired by a child’s inflatable toy, sold at Christie’s on Wednesday night for $91.1 million with fees, breaking the record at auction for a work by a living artist, set just last November by David Hockney.

Robert E. Mnuchin, an art dealer and the father of Treasury Secretary Steven Mnuchin, made the winning bid for Mr. Koons’s 1986 “Rabbit” from an aisle seat near the front of the salesroom. He was seated near Peter Brant, the collector and private museum-owner, and Jeffrey Deitch, the dealer.

It was the ultimate prize among six works offered at Christie’s from the collection of the magazine publisher S.I. Newhouse Jr., who died in 2017. Estimated to raise at least $50 million, this sculpture, made in an edition of three and one artist’s proof, was the last example left in private hands, according to Christie’s.

[Update: Robert Mnuchin would rather not discuss his client (or his son).]

The price, surpassing the $90.2 million with fees achieved, again at Christie’s, for Mr. Hockney’s 1972 painting, “Portrait of an Artist (Pool With Two Figures),” confirmed how Mr. Koons’s metal sculptures have become the ultimate billionaire trophies produced during the contemporary art boom of the 1980s and ’90s. The Hockney milestone came at the expense of Mr. Koons, the previous record-holder, whose “Balloon Dog (Orange)” sold at auction for $58.4 million in November 2013.