Born in Boston, Massachusetts, Kay Hanley 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 its 2008–2009 reunion tour). The band name was conceived by Hanley, in reference to a pen pal from her youth. In 1997, she began working on other projects, beginning with the role of Mary Magdalene in the Boston Rock Opera's performance of Jesus Christ Superstar.
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.” Upon the split of Letters to Cleo, she embarked upon a solo career. In 2002, she released Cherry Marmalade, her first solo album. 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.”