Sloan Simpson, the beautiful young fashion model who married New York's handsome postwar Mayor, William O'Dwyer, divorced him after his term as Ambassador to Mexico and then became a darling of the jet set as the ruler of Acapulco society, died yesterday at her home in Dallas. She was 80.

Friends said the cause was lung cancer, a disease that had previously cost her a lung but not her devotion to cigarettes.

If ever there was a match made in tabloid heaven, it was surely the one between the former policeman, who had succeeded Fiorello H. La Guardia as Mayor of New York in 1946, and the well-bred Texas charmer who had come to the city as one of the ''long-stemmed beauties'' of the John Powers modeling agency.

There was, for example, the cute way they met, in 1948 at the old Grand Central Palace convention center on Lexington Avenue at a fashion show in connection with the golden jubilee celebration of the consolidation of the five boroughs.

It may not have been love at first sight, but when the fashion executive Eleanor Lambert took the Mayor backstage ''to meet the models,'' as she put it yesterday, he had eyes only for one, a slender, dark-haired 33-year-old woman of such captivating beauty that friends insisted she was still turning heads more than four decades later.

The Mayor, at any rate, was captivated on the spot. Never mind that at 59, the widowed Mayor was old enough to be her father. For all his graying hair, he was a strikingly handsome man. And if there had ever been any doubts about his legendary charm, he dispelled them over the next few months as he proceeded to captivate her right back.

As the romance blossomed and the whispers turned into headlines, the city's newspaper readers gradually learned that Miss Simpson was no ordinary model. Her mother, the former Eleanora Laurenson Myer, was a legendary Baltimore beauty, and her father, also Sloan Simpson, who had left Harvard to follow Teddy Roosevelt up San Juan Hill, was the son of a wealthy cattleman who had once commanded what was described as the world's largest herd and founded the First National Bank of Dallas, now NationsBank.

Miss Simpson, whose parents were divorced when she was 9, grew up in the exclusive Highland Park section of Dallas and later studied at the Sacred Heart Convent in Torresdale, Pa., Miss Beard's School in Orange, N.J., and Stephens College in Columbia, Mo.

Miss Simpson, who had a brief early marriage that ended in divorce, was described by friends as a renegade Catholic. But the Mayor, who had studied for the priesthood in Spain, was too serious about his religion -- and too dependent on Roman Catholic voters -- to marry a divorced woman, and when Miss Simpson obtained an annulment, his friends knew something was up.

Even so, it was not until the Mayor had successfully won a second term in 1949 that the couple were married.

After the marriage, the new Mrs. O'Dwyer proceeded to supervise the first major decoration of Gracie Mansion.

Less than a year later, as mounting corruption investigations seemed to be closing in on the Mayor, he abruptly resigned and gratefully accepted appointment by President Truman as Ambassador to Mexico.

In Mexico City, the Ambassador and his wife won immense popularity, but strains soon appeared in the marriage, the result, Miss Simpson later said, of her husband's intense jealousy.

Whatever the cause, after the O'Dwyer stint as Ambassador ended with the Republican Presidential victory in 1952, Miss Simpson obtained a civil divorce in Mexico in 1953, then went off on a well-publicized sojourn to Spain, where she was photographed fighting bulls, repeatedly denied any other romantic interests and loyally insisted on her former husband's honesty. He died in 1964.

After the divorce, Miss Simpson tried her hand as an actress, receiving polite notices. ''Tolerable,'' said one; ''Wears a trench coat with aplomb,'' read another.

She eventually settled in Acapulco, where her charm established her as the city's leading society figure at a time when the resort was a magnet for the rich and well born. Until she returned to Dallas in 1989, Miss Simpson, whose family fortune had been dissipated by the time of her father's death in 1946, operated a boutique in Acapulco and worked for Braniff Airlines.

Miss Simpson, who told friends she would never marry unless she found a man with wealth and a title, had her share of titled suitors and romances with wealthy men, but never found one with both qualifications.

She leaves no immediate survivors.

Sloane Simpson