Pierre Bergé, who has died aged 86, was crucial to the 20th-century change in couture from a craft enterprise to an international megabusiness. His Napoleonic belief in his destiny was not focused on fashion until, at 29, he met at dinner Yves Saint Laurent, the fragile prodigy aged 22 who had suddenly inherited the house of Christian Dior. It was love at first conversation, about everything but fashion, so intense that Bergé forthwith left the artist Bernard Buffet, whose amanuensis he had been for years.

From then on, it was all about Yves. When Saint Laurent broke down during a rough first month as an army conscript, Bergé tracked him to a military hospital, and used his formidable energy and social aptitude to gain access. He became Saint Laurent’s support, suing the house of Dior for damages (Marc Bohan had taken Saint Laurent’s job) to fund an independent YSL atelier to which Dior personnel defected.

Bergé grasped that Paris couture was ailing, its houses’ commercial ventures too tentative when demographic and economic forces were opening up markets. He persuaded, or browbeat, Saint Laurent (Bergé alternated as hard and soft cop) into ready-to-wear in 1966, a financial and critical success.

Perhaps their compatibility – also combatability, given their opposite temperaments – came from outsidership. Saint Laurent was from Algeria, Bergé from the Île d’Oléron, an offshore island in the Bay of Biscay. Both had scant formal education and dreamed of Paris. At 17, Bergé left his father, Pierre, a civil servant, and mother Christiane (nee Sicard), to make his fortune in the capital. His accounts of his rise sound like a young-man-on-the-make from a Balzac novel; he traded in old books (collecting rare volumes all his life) and at 19 founded a shortlived anarchist magazine, La Patrie Mondiale.

Facebook Twitter Pinterest A board advertising the auction of the Yves Saint Laurent and Pierre Bergé collection in Paris in 2009. Photograph: Remy de la Mauviniere/AP

However, Bergé’s gift lay in meeting persons of consequence, by contrivance or accident: he claimed that the poet Jacques Prévert fell out of a window on to his head, that he shared a cell with the writer Albert Camus after a demo. Through Buffet he knew Jean Cocteau, through Cocteau Dior – et voilà! – Saint Laurent.

YSL ready-to-wear was merely Bergé’s initial bold move; as company president, he sold its rights in 1971 for capital to invest in a different way of staging couture shows, more like rock concerts, with clothes as loss-leaders to promote licensed accessories and perfumes. (YSL made enough to buy back the rights in 1973.) Bergé made complex deals when the luxury conglomerates of today had not yet managed their first merger; in 1986 he sold 25% of YSL for enough to buy Charles of the Ritz, which owned rights to Saint Laurent perfumes and cosmetics.

In 1989, the YSL group, with its Bergé–generated internal synergy, was the first designer house listed on the Paris Bourse, oversubscribed by 27 times. Those who got shares did exceptionally well for a while, but Bergé did better. He was fined 1m francs for insider trading, selling shares just before an announcement of plunging profits, while the pharmaceutical company Elf Sanofi paid over the market rate for the 44% of the capital held by Saint Laurent and Bergé before purchasing the group in 1993. Sanofi sold it on to Gucci in 1999, still with the vestigial involvement of Saint Laurent and Bergé, who did not leave as president until 2002.

While together, the couple created museum-like apartments in Paris and New York, a Normandy chateau with every room named after a Proust character, a villa and the glorious Jardin Marjorelle in Marrakesh. They split personally in 1976, when Saint Laurent’s perma-depression veered towards drink, drugs, and seclusion, although Bergé, patient with Saint Laurent despite being a prowling, growling panther, claws out, towards everyone else, kept his faith as well as the business going. They lunched daily, and Bergé, who respected Saint Laurent’s creativity, if not his metier (“a man of exceptional intelligence practising the trade of an imbecile” was his description) used his arts network to promote exhibitions of Saint Laurent. Showcasing a living designer was a novelty, and Bergé went wide with it, to museums in Beijing, Moscow and New York.

Bergé never ceased buzzing around the arts, owning the Théâtre de l’Athénée-Louis Jouvet, with weekly recitals by the best voices in town. As a lifelong socialist, albeit of the “caviar left”, supporting François Mitterrand, he refused a Mitterand cabinet post but accepted the presidency of the Paris Opera, in charge of the new Bastille Opera House, Palais Garnier and Salle Favart, from 1989 to 1994. There he vented even more ferocious temperament than he had in fashion, sacking musical director Daniel Barenboim for “too little work for too much reward”, and disputing with ballet director Rudolf Nureyev over his international schedule. Leading conductors refused to enter the Bastille, senior staff escaped it; the theatres were in turmoil. Bergé delivered far fewer productions than promised, and most were failures.

Mitterrand, a close friend of Bergé, appointed him an officer of the National Order of Merit (1987) and he was later appointed a grand officer of the Légion d’Honneur (2015). In 2010 he was one of the new investors who bought a controlling stake in the newspaper Le Monde.

Bergé went through a ceremony with Saint Laurent to become legal civil partners shortly before the designer died in 2008, and after disposed of their art collections, reserving only Jardin Marjorelle as their joint memorial, with a Saint Laurent museum.

This year Bergé married his long-term partner, the Jardin’s director Madison Cox, who survives him.

• Pierre Bergé, businessman, born 14 November 1930; died 8 September 2017