[3] His cousin, Élisabeth, Countess Greffulhe (1860–1952), was one of Marcel Proust's models for the Duchess of Guermantes in À la recherche du temps perdu.

Montesquiou had a strong influence on Émile Gallé (1846–1904), a glass artist with whom he collaborated, and from whom he commissioned major works, and from whom he received hundreds of adulatory letters.

The portrait Arrangement in Black and Gold: Comte Robert de Montesquiou-Fezensac was painted in 1891–92 by Montesquiou's close friend, and model for many of his eccentric mannerisms, James Whistler.

One author provides the following written portrait of Montesquiou:[6] Tall, black-haired, Kaiser-moustached, he cackled and screamed in weird attitudes, giggling in high soprano, hiding his black teeth behind an exquisitely gloved hand—the poseur absolute.

[12] Montesquiou likely contributed to Oscar Wilde's The Picture of Dorian Gray, and was a main basis of the homosexual character Baron de Charlus in good friend Marcel Proust's In Search of Lost Time – particularly its 1921 fourth part, Sodom and Gomorrah[9] – at which point he was effectively outed to the world.

[13] In 1885, he began a close long-term relationship with Gabriel Yturri (March 12, 1860 – July 6, 1905), a South American immigrant from Tucumán, Argentina, who became his secretary, companion and lover.

[3][14] In his biography, Philippe Jullian proposes that the Moberly–Jourdain incident in 1901, in which Charlotte Anne Moberly and Eleanor Jourdain claimed to experience time travel in the grounds of the Petit Trianon, is explained by their stumbling into a rehearsal of one of Montesquiou's Tableaux Vivants, with his friends (one possibly transvestite) dressed in period costume.