François Demachy was born in Cannes in 1949 and grew up in nearby Grasse, where his father ran a pharmacy — an early, unglamorous introduction to raw materials that steered him away from the dentistry and physiotherapy studies he initially pursued. He began training in perfumery around 1971 at the Charabot school in Grasse, working his way up from handling raw materials to composition over roughly five years.

From Charabot he moved to Chanel, where he spent more than two decades and rose to become Director of Research and Development, working across fragrances for Chanel itself as well as licensed lines including Bourjois, Emanuel Ungaro, and Tiffany & Co. In 2006 he left Chanel for Christian Dior, taking the newly created title of Perfumer-Creator and also serving as Director of Development for LVMH's perfumes and cosmetics division — the house's first perfumer to hold a dedicated, publicly credited creative role rather than working anonymously behind the brand.

At Dior, Demachy composed or substantially reworked many of the house's best-known scents over a fifteen-year run, including Sauvage (2015), revisions of Dior Homme and the Fahrenheit line, and a reimagining of Miss Dior in 2011 that saw the original renamed Miss Dior Original. He has described his approach as instinctive rather than formulaic, crediting a lifelong preference for florals — rose and jasmine especially, drawn from the fields around Grasse — and a habit of personally sourcing key raw materials, such as specific bergamot for Sauvage and sandalwood for Joy by Dior, from their countries of origin.

Demachy stepped back as Dior's in-house Perfumer-Creator in 2021, handing the role to Francis Kurkdjian, and in June 2022 The Fragrance Foundation honored him with its Lifetime Achievement Perfumer award. Across his career he is credited with well over a hundred fragrance creations for Dior and other LVMH-linked houses.