Denis Flageollet was born in 1962 in Gérardmer, in the Vosges region of northeastern France, into a family of watchmakers stretching back four generations. After completing his secondary education with a focus on the sciences, he left for Switzerland to train as a watchmaker and micromechanic at the Technicum du Locle. He rounded off his training as a watchmaking technician at the Watch Museum of Le Locle, restoring antique clocks and timepieces — work that would shape his philosophy for decades to come.

In 1982, Flageollet joined Michel Parmigiani's workshop, where he contributed to the restoration of classic and highly complicated watches, and worked on the development of an ultra-thin perpetual calendar movement for Breguet. After a period working independently on restorations and unique commissions for private collectors, he co-founded Techniques Horlogères Appliquées (THA) in 1989 alongside François-Paul Journe, Vianney Halter, and others. At THA, he served as technical and production director, overseeing the development of calibres and components for major brands. Notable projects included the Cartier Monopoussoir CPCP chronograph and the Breguet Sympathique clock.

In 2002, Flageollet co-founded De Bethune with collector David Zanetta in L'Auberson, near Sainte-Croix in the Swiss Jura. He serves as the brand's creative and technical director, and is the driving force behind its distinctive aesthetic and mechanical innovations. Under his direction, De Bethune has pioneered the use of heat-treated titanium — producing the brand's signature deep blue — developed a spherical moonphase display, advanced silicon escapement components, and introduced a three-point shock absorption system for the balance wheel. The DB28, winner of the Aiguille d'Or at the 2011 Grand Prix d'Horlogerie de Genève, is widely considered one of the most important independent watch designs of the modern era.

Flageollet is known for approaching watchmaking as a materials science as much as a craft — forging components by hand in his personal atelier between L'Auberson and Sainte-Croix, and drawing on centuries of horological history to inform thoroughly contemporary solutions. He has collaborated with ECAL and the EPFL on education initiatives, and in 2023 oversaw De Bethune's acquisition of Manufacture Reuge, the historic Sainte-Croix maker of music boxes and automata, as part of a broader commitment to preserving mechanical art traditions.