Kay Hanley was born in Dorchester, Massachusetts. She grew up diagonally across the street from Donnie and Mark Wahlberg. She was a member of Letters To Cleo from 1990 to 2000 (the length of the band's existence, with the exception of their 2008-2009 reunion tour). The band name was conceived by Hanley in reference to a pen pal named Cleo that she had when she was younger. In 1997, she began working on other projects, beginning with a role in the Boston Rock Opera's performance of Jesus Christ Superstar as Mary Magdalene.
In 1999, Hanley made a cameo as herself in the film 10 Things I Hate About You, singing a cover version of Nick Lowe's "Cruel to Be Kind". Towards the end of Letters to Cleo's career, she began performing with her husband and fellow Letters To Cleo member Michael Eisenstein outside of the band. Around the same time, she gave birth to their daughter, Zoe.
Upon the split of Letters to Cleo, Hanley embarked upon a solo career. In 2002, she released her first solo album, Cherry Marmalade. That same year, she appeared on the Dropkick Murphys/Face to Face split CD, providing guest vocals on the original version of the Dropkick Murphys song "the Dirty Glass".