Gens was born in Orléans, France, on April 19, 1966, into a family with mainly medical backgrounds (her father was a prominent doctor). She began singing in a choir as a child but, choosing to study English, planned to become an interpreter. In her teens, however, she shifted her focus to music and went on to win first prize for early music at the Paris Conservatory. Conductor William Christie introduced her to the Baroque repertory, and in 1986, she scored her first success with him when she appeared with Les Arts Florissants under his direction.
Gens' 1994 portrayal of the Countess in Mozart's Marriage of Figaro received wide acclaim and catapulted her to international stardom. By this time, her recordings of The Fairy-Queen and Rameau's Castor et Pollux, made several years earlier, had also garnered positive notice. Easily among her most notable recordings was the René Jacobs-led 1999 release of Così fan tutte on Harmonia Mundi. In 2001, Virgin Classics released her rendition of the Berlioz song cycle Les nuits d'été, with the Orchestre de l'Opéra national de Lyon led by Louis Langrée. In 2004, she appeared in Debussy's Pelléas et Mélisande at the Deutsche Oper Berlin.
Gens has personally stated a preference for Mozart and, besides singing the Countess and Fiordiligi, has sung Donna Elvira (Don Giovanni) and Vitellia (La Clemenza di Tito). She considers Mozart the perfect bridge for her other favorites, Baroque repertory and French music from the 19th and early 20th centuries. Gens has sung with some of the most important European conductors throughout her career, including Claudio Abbado, Philippe Herreweghe, and Marc Minkowski. Besides Harmonia Mundi and Virgin Classics, Gens has recorded for L'Oiseau-Lyre, Erato, and Archiv. In 2006, Gens made the first of her Tragediennes solo recordings, with a third volume released in 2011.
Gens has continued to record Baroque repertoire in the 2010s, taking the leading role in a recording of Rameau's "ballet heroïque" Les fêtes de Polymnie in 2015. Much of her output during this period, however, has consisted of weightier French roles of the 19th century, including works by Berlioz, Gounod, and Saint-Saëns. Not content with big-production starring roles, Gens made several recordings of rare operas for the Ediciones Singulares label. In 2015, she made her first recording for the Alpha label: Néère, a collection of French art songs, was met with acclaim. Gens released Visions, a collection of ambitious and often obscure French arias and cantatas on a shifting border between sacred and profane that only she could have navigated in 2017. The French government has conferred upon Gens the honors of Chevalier of the Legion of Honor and Chevalier of Arts and Letters. In 2020, Gens issued the solo album Nuits and was heard in starring roles on recordings of Lully's Armide and Offenbach's Maître Péronilla. ~ Robert Cummings & James Manheim, Rovi