Sager was born in Manhattan to Anita Nathan Bayer and Eli Bayer. Her family was Jewish. She graduated from New York University, where she majored in English, dramatic arts and speech. She had already written her first pop hit, "A Groovy Kind of Love", with Toni Wine, while still a student at New York City's High School of Music and Art. It was recorded by the British invasion band the Mindbenders, whose version was a worldwide hit, reaching #2 on the Billboard Hot 100. This song was later recorded by Sonny & Cher, Petula Clark, and Phil Collins, whose rendition for the film Buster reached number one in 1988. She also had a career as a singer, including her 1977 Australian number one single "You're Moving Out Today", which also reached Number 6 in the UK singles chart in June 1977.