After a period working for the British fashion designer Lucile, Molyneux, a former illustrator, opened his fashion house in Paris, 14 rue Royale, in 1919 and went on to dress European royalty, British high society, the actresses Greta Garbo, Gertrude Lawrence, Margaret Leighton, and Vivien Leigh, and the interior decorator Syrie Maugham. His followers included Christian Dior and Pierre Balmain.
During World War II, he moved his firm to London for the duration of the conflict and returned to Paris in 1946. In 1965, when the designer came out retirement, Time magazine described him as "the Parisian equivalent of Manhattan's Mainbocher, a classicist devoted to the soft look and tailored line."
Though he retired in 1950 to take up painting, leaving his fashion house in the hands of designer Jacques Griffe, Molyneux returned to fashion in 1964, when he opened Studio Molyneux, a high quality ready-to-wear line that received mixed reviews. He retired again in 1969, but Studio Molyneux continued under the direction of his cousin John Tullis until it closed in 1977.
The Molyneux trademark is owned by a company in France, and though the fashion component of the firm remains dormant, the firm still produces scents, such as Captain (1975), Quartz (1978), Le Chic, Vivre, and I Love You.
Molyneux married, in 1923, as his first wife, (Jessie) Muriel Dunsmuir (1890-1951), one of the eight daughters of the Hon. James Dunsmuir, Premier of British Columbia. They divorced in 1924.
Molyneux served as an infantry captain with the British army during World War II, during which time he lost an eye in battle.