Born in New York City in 1947. In the 1960s, she attended New York's High School of Music and Art, where she began writing poems - as lyrics. In 1966, she came to the attention of Screen Gems president Don Kirshner, who signed her to his publishing company, where she soon was penning songs for such groups as The Monkees and The Mindbenders. In 1970, she wrote the lyrics for the off-Broadway show Georgy.
In 1975, Sager and Melissa Manchester co-composed "Midnight Blue", and "Don't Cry Out Loud". In 1978, Elektra issued her 'debut' album. Singer Leo Sayer had a hit with one of her tunes, "When I Need You". Sager next worked with husband, Marvin Hamlisch, on the Broadway musical They're Playing Our Song, which may have had some autobiographical content exploring the romantic entanglements of a songwriting team. In 1981, Sager's third LP "Sometimes Late at Night," had her biggest solo hit - the single "Stronger Than Before".
Regretfully, her marriage to Hamlisch ended in divorce, amd she began working with another composer, Burt Bacharach. Their 1981 song "Arthur's Theme (Best That You Can Do)", - a hit for singer Christopher Cross, earned them an Academy Award. The next year, Sager and Bacharach, were married, and though this marriage also, sadly, ended in divorce (1991), their partnership was very fruitful. The song "That's What Friends Are For," originally written in 1982 for the comedy NIGHT SHIFT, is perhaps their most notable. For singer Neil Diamond, Sager, and Bacharach wrote the hit "Heartlight". For a Patti Labelle /Michael McDonald duet, they wrote "On My Own". In 1988 the team scored the soundtrack for the sequel ARTHUR 2: ON THE ROCKS, and also wrote some tracks for Barbra Streisand's LP "Till I Loved You."
Since her divorce from Bacharach, Sager has continued to work, writing songs for such films as BEETHOVEN'S 2ND and JUNIOR. In 1996, Sager married Robert Daly.