Jay Goldberg, a former boxer, who became a comedian and a colorful lawyer who defended celebrity clients and a bevy of mobsters, died on Dec. 5 at his home in Bridgehampton. He was 89.

Mr. Goldberg relished his role as a gladiator, first as a 6-foot-2, 140-pound welterweight who retired when a knockout in the third round ended his 10-0 winning streak, then as a “killer” in the courtroom, who also occasionally filled in at the Copacabana nightclub in Manhattan (his aunt knew the owner), sharing Mark Twain’s faith that a laughing jury seldom convicts.

Once a mobster whom he represented and who had pleaded guilty was about to be sentenced by a federal judge. As Mr. Goldberg stood beside his client in a packed courtroom, the judge asked whether the defendant “should suffer economic loss for his criminal activity.”

Judge,  “I have taken care of that already, ” Mr. Goldberg said.

As a private defense lawyer, Mr. Goldberg attracted a long list of high-profile clients, including Miles Davis, Bono, Johnny Cash, the Hells Angels, Carl Icahn, Charles Rangel, Meyer Lansky, Sean Combs, Willie Nelson, The Rolling Stones, and Donald J. Trump. He was the author of five books, notably a memoir, The Courtroom Is My Theater. In a New York Times interview in 1997, Mr. Goldberg said, “It’s the only place where you get to emote and try to convince juries that black is white.”