Long before Jennifer Lopez and Eva Longoria, there was Rita Hayworth. The dancer-actress, born Margarita Carmen Cansino, would have been 100 on Wednesday, and even fans of classic Hollywood may not realize how extraordinary her career was. She was the No. 1 box office star of Columbia Pictures in the 1940s, she was Fred Astaire’s favorite dancing partner, and U.S. G.I.s had pinups of her around the globe during WWII. These are especially impressive in an era when Latino-Hispanic children were still in segregated schools and only a decade after America’s “repatriation” program shipped 2 million Mexicans across the border, claiming they were “stealing” American jobs.

Work dried up for Hayworth in the 1960s, due to occasional slurred speech and memory problems. Hollywood assumed she was alcoholic, but in 1980 she was diagnosed with Alzheimer’s, bringing worldwide awareness to the little-known disease. Her greatest legacy may be the annual Rita Hayworth Galas, which have raised $75 million for the Alzheimer’s Association.

Viewers of old movies know Hayworth from the 1944 musical “Cover Girl,” the film noir “Lady From Shanghai” (directed by her second husband Orson Welles), and other classics. But her signature role was in “Gilda.” In the March 13, 1946, review, Variety said, “Rita Hayworth asserts herself definitely as a dramatic star” in her first major non-musical role.

She was born in Brooklyn and began taking dance lessons at age 3. The family moved to Southern California and she began performing at age 12 with her Spanish father in an act called the Dancing Cansinos.

Discovered by Winfield Sheehan of Fox Film Corporation, she got a small role in the 1935 film “Dante’s Inferno” at age 16. Other minor roles followed and in 1937, promoter Edward Judson married her and took over her career, arranging a seven-year contract with Columbia. Judson and Columbia topper Harry Cohn changed her hairline through electrolysis and gave her elocution lessons. They also changed her name to Rita Hayworth. The studio de-emphasized her Hispanic heritage, but they didn’t lie about it; since she had made 10 films under the name Rita Cansino, they couldn’t. (That’s in contrast with actress Merle Oberon, an Anglo-Indian star born in Bombay, but who always maintained that she was born in Tasmania and that her birth records were destroyed in a fire.)

After divorcing Welles in 1948, Hayworth wed Prince Aly Khan, whom Variety described as “one of the wealthiest men in the world, an international playboy and son of the spiritual leader of millions of Moslems.” Clearly, this was not a traditional Hollywood wedding.

In 1989, two years after Hayworth’s death at age 68, Viking published Barbara Leaming’s biography, alleging that most of Hayworth’s troubles stemmed from sexual and physical abuse from her father when she was young. (The New York Times review was headlined “What We Have Here Is a Very Sad Story.”)

It is definitely a sad story, but onscreen she was joyous. Near the end of “Gilda,” she performs a song and dance to “Put the Blame on Mame,” in which she somehow manages to be knockout sexy and to simultaneously laugh at the whole idea of being a sex symbol.

In the 21st century, fans have posted YouTube clips of her dancing skills, such as the “Rita Hayworth Jumps in the Line” montage and another clip with Astaire overlaid with a remix of the Elvis Presley hit “Bossa Nova Baby.” It’s clear why Astaire (and the audience) liked her so much.

Aside from her screen legacy, she achieved literary immortality thanks to the Stephen King novella “Rita Hayworth and Shawshank Redemption,” in which her pinup proves surprisingly important to the prisoners. (The movie retained her image, but eliminated her from the title. The movie also added the word “The” before “Shawshank.”)

Hayworth began to show symptoms of Alzheimer’s when in her 40s. She was diagnosed in 1980 and her daughter, Princess Yasmin Aga Khan, has worked hard since then to raise awareness. The 35th annual Rita Hayworth Gala will be held Oct. 23 at Cipriani in Manhattan.