Her marriage with Barry soon broke up, however, and a trip to France introduced her to Gallic pop star Serge Gainsbourg. The two eventually married, and Birkin lent her talents to Gainsbourg's 1969 recording of the erotic pop song "Je T'Aime...Moi Non Plus." Originally released by Fontana Records in Britain, the single was soon dropped by the label; reissued on the Major Minor imprint, it hit number one in England late that year despite a radio ban. The collaborative LP Je T'Aime...Moi Non Plus soon followed, though Birkin spent much of the early '70s working in films. She appeared in much exploitation fare, including Sex Power, Romance of a Horse Thief, and Don Juan 73, the latter featuring her as the same-sex lover of Brigitte Bardot. With help from Gainsbourg, she recorded 1975's Lolita Go Home and 1978's Ex Fan des Sixties, gaining hits in France, if not in England.

Her marriage to Gainsbourg dissolved in 1980 (their daughter, Charlotte Gainsbourg, became a singer herself, and made a bit of controversy recording the single "Lemon Incest" with her father), and Birkin later married French director Jacques Doillon. She continued performing, acting, and making music, mostly directed to a French audience, until 2006 when she released Fictions. The album included both a Tom Waits and a Neil Young cover along with new material from songwriters Neil Hannon of Divine Comedy, the Magic Numbers, Beth Gibbons, and Rufus Wainwright. The self-penned Enfants d'Hiver arrived in 2008.