Born in 1951 in New York, Ian entered the American folk music scene while still a teenager in the mid-1960s. Most active musically in that decade and the 1970s, she has continued recording into the 21st century. She has won two Grammy Awards, the first in 1975 for "At Seventeen" and the second in 2013 for Best Spoken Word Album, for her autobiography, Society's Child, with a total of ten nominations in eight different categories.

As a child, Ian admired the work of folk pioneers such as Joan Baez and Odetta. Starting with piano lessons at the age of two (at her own insistence), Ian, by the time she entered her teens, was playing the organ, harmonica, French horn and guitar. At the age of 12, she wrote her first song, "Hair of Spun Gold," which was subsequently published in the folk publication Broadside and was later recorded for her debut album. In 1964, she legally changed her name to Janis Ian, taking her brother Eric's middle name as her new surname.

At the age of 14, Ian wrote and recorded her first hit single, "Society's Child (Baby I've Been Thinking)", about an interracial romance forbidden by a girl's mother and frowned upon by her peers and teachers. Produced by George "Shadow" Morton and released three times from 1965 to 1967, "Society's Child" became a national hit upon its third release after Leonard Bernstein featured it in a CBS TV special titled Inside Pop: The Rock Revolution.

Ian relates on her website that, although the song was originally intended for Atlantic Records and the label paid for her recording session, Atlantic subsequently returned the master to her and quietly refused to release it. Ian relates that years later, Atlantic's President at the time, Jerry Wexler, publicly apologized to her for this. The single and Ian's 1967 eponymous debut album (which reached #29 on the charts) were finally released on Verve Forecast. In 2001, "Society's Child" was inducted into the Grammy Hall of Fame, which honors recordings considered timeless and important to music history. Her early music was compiled on a double CD entitled Society's Child: The Verve Recordings in 1995.

Other artists have recorded Ian's compositions, including Roberta Flack, who had a hit in 1973 with Ian's song "Jesse". Ian's own version is included on the 1974 album Stars (the title song of which has also been oft-covered, including versions by Joan Baez, Shirley Bassey, Cher, Nina Simone and Barbara Cook). Richard Barone recorded Ian's song Sweet Misery on his album Sorrows & Promises: Greenwich Village in the 1960s in 2016. She continues to tour in both the US and the UK.

Ian's mother Pearl Fink was diagnosed with multiple sclerosis in 1975. Because of this, Ian and her brother persuaded their mother to pursue her lifelong dream of going to college. Fink eventually enrolled in Goddard College's adult education program and ultimately graduated with a master's degree. After Fink's death in 1997, Ian decided to auction off memorabilia to raise money to endow a scholarship at Goddard specifically for older continuing education students, which became the Pearl Foundation, a 501(c)(3) public charity. At the end of each year, 90% or more of funds raised from sale of merchandise, donations from fans and contributions from Ian herself are disbursed to various educational institutions to fund scholarships. By 2016, it had contributed over $900,000 in scholarship funds.

"Fly Too High" (1979), produced by disco Producer Giorgio Moroder, was Ian's contribution to the Soundtrack of the Jodie Foster film Foxes and was also featured on Ian's 1979 album Night Rains. It also became her first international hit, reaching #1 in many countries, including South Africa, Belgium, Australia, Israel and the Netherlands, and going gold or platinum in those countries as well as charting in the UK. Another country where Ian has achieved a high level of popularity is Japan: Ian had two Top 10 singles on the Japanese Oricon charts, "Love Is Blind" in 1976 and "You Are Love" in 1980. Ian's 1976 album Aftertones also topped Oricon's album chart in October 1976. "You Are Love (Toujours Gai Mon Cher)" is the theme song of Kinji Fukasaku's 1980 movie Virus. She cut several other singles specifically for the Japanese market, including 1998's "The Last Great Place".

Ian took acting lessons from noted acting coach Stella Adler in the early 1980s to help her feel more comfortable on stage, and she and Adler remained close friends for the rest of Adler's life. In December 2015, Ian appeared in the series finale of HBO comedy series Getting On playing a patient who refused to stop singing.

From 1982–92, Ian continued to write songs, often in collaboration with then-songwriting partner Kye Fleming, which have been covered by Amy Grant, Bette Midler, Marti Jones and other artists. She released Breaking Silence in 1993 and also came out as a lesbian. The album, Folk Is The New Black, was released jointly by her Rude Girl Records label and the Cooking Vinyl labels in 2006.

Ian started "Rude Girl Records, Inc. and its publishing arm, Rude Girl Pub., [on] January 2, 1992". "From 1992 to the present, RGR has steadily grown, with its current ownership of Janis Ian masters up to twenty albums and DVD's overseas, and a slightly smaller number in North America. The Rude Girl label oversees the production of Janis' newer work, and in the case of older work, its re-mastering and the re-creation of the original artwork."

Ian married Portuguese filmmaker Tino Sargo in 1978 and the two divorced in 1983. Details of Sargo's physical and emotional abuse were discussed in Ian's autobiography. After moving to Nashville, she met Patricia Snyder in 1989. Ian came out as a lesbian in 1993 with the worldwide release of her album Breaking Silence. Snyder and Ian married in Toronto on August 27, 2003. Ian has a stepdaughter and two grandchildren with Snyder.

Ian has been a regular columnist for the LGBT news magazine The Advocate and contributed to Performing Songwriter magazine from 1995–2003. On July 24, 2008, Ian released her positively-received autobiography Society's Child (published by Penguin Tarcher) . An accompanying double CD, The Autobiography Collection, has been released with many of Ian's best loved songs.

On October 23, 2011, Ian performed with Ryan Adams and Neil Finn on BBC Four's Series 2 Episode 4 of the series, Songwriters' Circle. The night resulted in controversy, ending with an awkward exchange between the performers when there seemingly was confusion between the songwriters about who was to perform and join in on each other's songs.

At age 16, Ian met Comedian Bill Cosby backstage at a Smothers Brothers show where she was promoting Society's Child. Since Ian was underage, she was accompanied by a chaperone while touring. After her set, she had been sleeping with her head on her chaperone's lap (an older female family friend). According to Ian in a 2015 interview, she was told by her then manager that Cosby had interpreted their interaction as "lesbian" and as a result "had made it his business" to warn other television shows that Ian wasn't "suitable family entertainment" and "shouldn't be on television" because of her sexuality, thus attempting to blacklist her. Although Ian would later come out as lesbian, she states that at the time of the encounter with Cosby she had only been kissed once, in broad daylight at summer camp.