Catherine de Károlyi was born Catherine Pólya in Budapest on 1 September 1919, the only daughter of the painter, poster artist and caricaturist Tibor Pólya. Gifted at drawing and drawn to fashion from girlhood, she married the young Count Étienne Károlyi in 1945, and with him she left Hungary in 1947, after the Communist Party had crushed its rivals and the family's estates had been collectivised. The plan was to continue to Argentina; instead the couple remained in France, where they lived with refugee status.
In Paris she worked first as a model and then as a sketcher and designer for the couturier Robert Piguet. After his death she designed for Jacques Fath and for Christian Dior, and she later opened a house of her own under the name by which she had become known in fashion, Catherine de Károlyi. She kept that professional name after her divorce in 1953 and her second marriage, to the writer and screenwriter Raymond Assayas, known as Jacques Rémy, through whom she acquired French nationality.
Her lasting place in fashion history was made at Hermès. In 1967 the house, then still primarily a saddler and maker of leather goods and silk, entrusted her with the women's wardrobe, and she designed its first women's ready-to-wear collection. Until 1980 she continued to design clothing and accessories for the house: bags, belts and silk squares among them. One of her designs, the belt buckle in the form of a capital H, became one of the most recognisable signatures Hermès possesses and remains in production today.
Her work at Hermès coincided with a broader transformation of Paris fashion, as the great houses began to look beyond made-to-measure towards ready-to-wear. Her clothes for the house were understated and practical, conceived as a natural extension of its accessories rather than a departure from them; dresses from around 1970 attributed to her direction, in jersey and printed silk, still appear on the vintage market.
Her career is recalled in Fashion Mix, the exhibition and catalogue on foreign-born designers in French fashion produced by the Musée national de l'histoire de l'immigration in Paris, and in the archive of OFPRA, the French office for refugees, which counts her among its notable former refugees. She was the mother of three sons, among them the filmmaker Olivier Assayas and the journalist and writer Michka Assayas; her eldest son, Georges Károlyi, presides over the Joseph Károlyi Foundation and was appointed Hungary's ambassador to France in 2014.
Catherine de Károlyi died on 30 October 2006, at the age of 87.