The cause was cancer, her daughter, Allegra Clegg, said.

Tall and authoritative, Ms. Mounsey put such a stamp on roles — even ones she did not create, as in Balanchine's "Prodigal Son" and "Serenade" — that people still recall her delivery of certain moments after more than 50 years.

Ms. Mounsey joined City Ballet in 1949, dancing in many of its early performances of Balanchine's "Serenade" (as the Dark Angel) and "The Four Temperaments." Although she was not the first Siren when Balanchine revived his 1929 ballet "Prodigal Son" in 1950, Ms. Mounsey became the role's first classic interpreter on American soil. She had studied the role with its originator, Felia Doubrovska, and decades later coached it on film for the George Balanchine Foundation.

With her spectacular height — she was over 6 feet tall on point — long legs and cool eroticism, she came to epitomize the character for a generation.

For Balanchine, she also created supporting solo roles in "La Valse" and "Swan Lake" (both in 1951) and the Spanish dance in "The Nutcracker" (1954). For Robbins, she created the roles of the Queen in "The Cage" (1951), the Harp in "Fanfare" (1953) and the Wife in "The Concert" (1956). All these ballets remain in the repertories of City Ballet and other companies.

Yvonne Louise Leibbrandt was born in September 1919 on a farm outside Pretoria, South Africa, where she received her early ballet training. Her pursuit of a ballet career took her to Britain in 1937. There she danced on tour with the Carl Rosa Opera Company and stayed in London to train with Igor Schwezoff. Too tall to be of the proper type for the British ballet of that era, she auditioned instead for the Ballet Russe de Monte Carlo, successfully.

But she had scarcely traveled to Monte Carlo to join that troupe when World War II broke out. She returned briefly to South Africa before heading to Australia and soon joined the Original Ballet Russe, a separate troupe, taking the stage name Irina Zarova. When the company came to New York in 1940, Balanchine was immediately struck by her and cast her in his first setting of Stravinsky's violin concerto "Balustrade" (1941).

At one point during the war as she toured with the Original Ballet Russe Ms. Mounsey found herself stranded in Cuba. She turned the stay to her advantage, becoming a successful nightclub dancer there. In 1945, when she took her act to Mexico City, she met Balanchine again and accepted his invitation to perform there with a group of his New York dancers. In a subsequent return to South Africa, where she performed to acclaim, she married Duncan Mounsey, an American.

After her time at New York City Ballet, Ms. Mounsey returned to South Africa in 1959 and, with Faith de Villiers, founded the Johannesburg City Ballet. It later became the PACT Ballet, named for the newly instituted Performing Arts Council of the Transvaal.

Ms. Mounsey moved to Los Angeles in 1966. The next year she founded the Westside School of Ballet in Santa Monica, which became one of Southern California's most influential ballet centers. There she helped train many students who went on to be leading American dancers, among them Jock Soto, Andrew Veyette and Tiler Peck.

Her marriage to Mr. Mounsey ended in divorce, as did a second marriage, to Hall Hughey. A third marriage, to Kelvin Clegg, a childhood sweetheart, ended with his death in the 1990s. Besides her daughter, Ms. Mounsey is survived by her stepsons, Stephen and Christopher Clegg, and a grandson.