Roberta Peters, who debuted at the Metropolitan Opera at age 20 on five hours’ notice and became a reigning soprano of her era, delighting audiences for decades with performances on stage, in commercials and on “The Ed Sullivan Show,” died Jan. 18 at an assisted living facility in Rye, N.Y. She was 86.

The cause was Parkinson’s disease, said her son Paul Fields.

Ms. Peters’s maiden entrance at the Met, the venerable opera house in New York, could scarcely have been more dramatic, even if it had been scripted by a librettist.

Groomed since childhood for a career on stage — although she had not yet sung on one — she was slated to appear at the Met as the Queen of the Night in “The Magic Flute” in 1951.

But on Nov. 17, 1950, soprano Nadine Conner, who was to sing Zerlina in that evening’s performance of “Don Giovanni,” came down with food poisoning, according to the New York Daily News. (Ms. Peters and her family were already looking forward to attending the performance, in the standing-room section.)

Ms. Peters in her dressing room on Nov. 17, 1950, the night of her Metropolitan Opera debut. (Sedge LeBlang/Metropolitan Opera Archives)

Rudolf Bing, the Met’s newly installed general manager, had a crisis on his hands and called Ms. Peters to ask if she could relieve it.

“Can you sing tonight?” he inquired, in a 3 p.m. phone call. With confidence that she recognized years later as extraordinary, Ms. Peters assured him that she could.

“We took the subway. We couldn’t get a cab,” she told the Associated Press in 1985. “It was the first time I’d ever sung professionally anywhere, and there I was, pushed out on the stage to sing at the Met.”

With that, Ms. Peters, the daughter of a hat maker and a shoe salesman, was transformed into Zerlina, the peasant. The young singer’s parents received upgraded box seats for the occasion.

“She was wonderful,” the conductor, Fritz Reiner, later told the New York Herald Tribune. “A really gifted girl. Her fine preparation should be a lesson to other young American singers. When the chance came, she was qualified.”

That performance marked the first of more than 500 appearances by Ms. Peters at the Met over 35 years. She also sang at the Vienna State Opera, at London’s Royal Opera House at Covent Garden, before U.S. presidents and on TV programs including Sullivan’s show, “The Voice of Firestone” and “The Tonight Show” with Johnny Carson.

In one unforgettable commercial, she belted the Chock full o’Nuts coffee jingle in full operatic attire — and full-on operatic vocal power. In another, for American Express, she flagged a cab, calling out “Taxi!” in her inimitable soprano.

Ms. Peters at her home in Purchase, N.Y., in 2000. (Jeff Geissler/AP)

Her most noted roles at the Met, besides her first two Mozartean parts, included Rosina in Rossini’s “The Barber of Seville,” Gilda in Verdi’s “Rigoletto” and Susanna in Mozart’s “The Marriage of Figaro.”

In 1955, she played Oscar in the performance of Verdi’s “A Masked Ball” in which Marian Anderson, the acclaimed African American contralto, made her long-delayed debut at the Met.

Although Ms. Peters preferred lighter soprano parts, she ventured into somewhat more dramatic music as the title character in Donizetti’s “Lucia di Lammermoor.”

“I’ve seen so many singers be at the Met five years and ruin their voices,” she said. “Your vocal cords are very delicate. If you sing parts that are too heavy, you push, and that can do harm or even ruin you. Once in a while, I had the urge to sing heavier roles, but I stopped myself in time.”

Ms. Peters’s care with her voice allowed her to sing well into her later years on the opera stage, as a recitalist and in musicals. In 1998, she received the National Medal of Arts.

Roberta Peterman — Peters was her stage name — was born in the Bronx on May 4, 1930. Her family quickly recognized her vocal talent.

“It was my mother who had the dream,” she told the AP, recalling her mother counseling that “one day if you work hard, you’ll make the Met.”

Ms. Peters’s grandfather was a headwaiter at Grossinger’s, the resort in the Catskills that catered to Jewish patrons, and arranged for her to audition before the tenor Jan Peerce, who sometimes sang there. Peerce confirmed the family’s appraisal of Roberta’s talents.

She left school to pursue musical training, which proved more rigorous than many traditional educations. She studied Italian, French and German. To cultivate her poise, she balanced a bag of gravel atop her head. To strengthen her diaphragm, she allowed Joseph Pilates, creator of the physical fitness regimen that bears his name, to stand on top of her.

“It was such an unusual teen age,” she told the New York Times. “I never went to a prom. I was taken out of school at age 13, and have no high school or college diplomas.”

Before her audition at the Met, she had learned 20 operas. During the tryout, Bing asked to hear her Queen of the Night aria again — and again, and again.

“It’s the most difficult aria, I think, ever written for a soprano,” she once said in an interview on CBS. In the darkness of the auditorium, “what I didn’t know was that he was calling in the stage directors and other conductors and all to hear me. And when I came off, he offered me a contract. So that could be like a Cinderella story except that I was prepared.”

She also credited her rise to the impresario Sol Hurok, whose life was recounted in the film “Tonight We Sing” (1953), in which she had a part.

Ms. Peters was briefly married to the operatic baritone Robert Merrill and continued performing with him after their amicable divorce. Her husband of 55 years, Bertram Fields, died in 2010. Survivors include two sons, Paul Fields of Bethesda, Md., and Bruce Fields of Westport, Conn.; and four grandchildren.

Ms. Peters owed her career at least in part to her availability, if not youthful derring-do, during a desperate moment at the Met. To the end, she stood ready to sing, whenever she was needed.

She told the Chicago Tribune in 1993, more than four decades into her career, “As long as people still want to come, I’m available!”