Jackie Kennedy's wardrobe designer

Oleg Cassini: Jackie Kennedy wrote to Oleg Cassini less than a month after her husband Jack was elected US president in 1960 asking: "Are you sure you are up to it, Oleg?" The "it" was the job of dressing the first lady. "Jackie had an eye," Cassini, who has died aged 92, explained long after, "but she had no means, never had any money."

That ended when her father-in-law, Joseph Kennedy, who had dabbled in Hollywood production and understood the voter value of imagery, recognised in Jackie a chance to enhance the Kennedy package. Joe did not care that Jackie's tastes were Francophile; once she was in the White House, her labels had to be American.

Joe called in Cassini, a family friend who had designed in Hollywood, and Cassini "talked to Jackie like a movie star, told her she needed a scenario as first lady". She sent him detailed lists of her wardrobe requirements, with references to models shown by Hubert de Givenchy in Paris - there were later debates among dress scholars over what was Cassini and what Givenchy. Cassini established an atelier to make 300 of the outfits Jackie wore in her 1,000 days as the president's wife. The bills went straight to Joe. Jack, grumbling, paid for the clothes Jackie bought directly from Paris.

Cassini described himself as almost French: he had been born in Paris, son of a count and grandson of the Russian ambassador to the US. The family did not care for their native Russia, and after the revolution did not return: his mother settled to a couture business in Florence. He studied art, sketched for Patou in Paris and had a playboy reputation. Like his brother Igor (who became a gossip columnist under the name Cholly Knickerbocker), Cassini relocated his charm, sportiness and lack of funds to New York in 1936, gravitating to Los Angeles.

He was working as a costume design assistant to Edith Head in Hollywood in 1940 when he met classy ingenue actor Gene Tierney. She thought him the most "dangerous-looking character" she had ever seen; he considered her perfect but for her "slightly bucked teeth, which only added to her allure . . . she came from a good family". They romanced dancing at Ciro's - he described them "surrounded by magnificent people in beautiful clothes", stressing the frocks rather than the folks.

Her good family disapproved of his womanising (he had divorced his first wife, the heiress Merry Fahrney) and continued cash shortage, so, in 1941, the couple eloped and married in Las Vegas. Cassini lost his job, and had to depend on his wife until, with America's entry in the second world war, he joined the US cavalry as an officer.

Then their daughter Daria was born hearing and speech impaired and diagnosed as "mentally retarded". This destabilised the couple. When Tierney met the young John Kennedy on a movie shoot in 1944, they began a relationship that only ended when he told her there was no possibility of marriage as he needed a politically acceptable wife. She was reconciled with Cassini (they had another daughter, Christine in 1948), but they finally divorced in 1952.

Her replacement was more elegant yet - Grace Kelly; Cassini saw her in the film Mogambo and whispered to a friend: "She is going to be my next romance." A few hours after, he sat down three feet away from her in a small restaurant. On each of the next 10 days he sent her red roses with a card signed "The Friendly Florist", until she agreed to a date. He claimed that they were engaged on the Riviera during the filming of To Catch a Thief, but she broke it off and married Prince Rainier of Monaco.

Cassini left Hollywood as the studios contracted in the early 1950s, and opened his own French and Italian inspired ready-to-wear business in New York. His brother had named the teenage Jacqueline Bouvier, Queen Deb of 1947. Cassini first met her in the El Morocco nightclub, five years before her marriage to Kennedy: he recalled her fit muscle tone, especially her upper arms in sleeveless gowns.

Joe Kennedy approached Cassini because he knew the designer would be discreet. As writer Pamela Clarke Keogh noted, Cassini not only dined chez Kennedy, he taught Jackie the twist, discussed the Kamasutra with Jack and once persuaded Jack to allow Jackie to wear a one-shouldered gown on a state occasion by comparing her with ancient Egyptian royalty. But Joe could never have guessed that the Cassini interpretation of Givenchy's silhouette with exaggerated details - buttons and pockets in scale with the salons of diplomacy - would be the first American look to be popular worldwide. When the garments were displayed decades later, critics thought it an armoured style, for all its camera-catching colours. The clothes were there to protect her vulnerability.

Cassini's greatest success was Jackie's dress for the inauguration gala in 1961: she had ordered a complicated outfit from Bergdorf Goodman, and also a plain gown of heavy satin from Cassini. "Now I know how poor Jack feels when he has told three people they can't be secretary of state," she wrote to Cassini. She chose to wear his creation for the gala and kept a photograph of herself in it, leaving her old home in the snow, in her White House dressing-room. Jack told her to sit forward in her car seat, and his driver to switch on the lights, so people could see her in that dress: the young, hopeful bride of the nation.

Cassini was not responsible for the pink suit she was wearing in Dallas, in 1963, when Jack Kennedy was assassinated: that was a Chanel copy.

After the president's funeral, Jackie dropped Cassini, understanding instinctively that his work had become demode. For the rest of the 20th century, the Jackie style was visible chiefly in the uniforms of air hostesses and hotel receptionists.

He stayed in the business, though, in middle-market menswear, profitable enough to buy him the old Tiffany estate at Oyster Bay Cove, which he shared with over 100 animals, including a stable of horses, 30 goats, one half-ton sow and two potbellied pigs. He had designed Jackie a leopard-pelt coat so influential that thousands of big cats were killed to make copies. When he realised what he had done, he converted to conservation.

He is survived by his third wife Marianne, and by his two daughters.

Oleg Cassini Loiewski: born April 11th, 1913; died March 17th, 2006