Born in Leominster, Massachusetts, she is the third child of The Kelly Family, a multi-generational pop group that achieved success in Europe in the 1990s.

While continuing to record and tour with the group throughout the 2000s, Kelly pursued her solo projects, resulting in three additional LPs and several singles to date.

In 1974, Kathy, Caroline, Paul, John, and Patricia formed The Kelly Kids to perform at children's parties and on the streets.

She is credited with writing Lonely, a song recorded in 1981 featuring lead vocals by John, the then-main vocalist, and Papa Dan.

The year 1994 marked a major breakthrough for the Kelly Family, as they began selling CDs in stores.

Throughout her career with the Kelly Family, Kathy contributed to 32 of their 35 albums and provided vocals on 106 songs, including live CDs, a total exceeded only by Paddy and John.

Kathy Kelly began her solo career by releasing a single titled You're Losing Me, which reached number 94 on the German charts.

After leaving the Kelly Family in 2000, Kathy released another album, Morning of My Life, featuring entirely new material.

Kathy also embarked on the Godspel Tour from 2009 to mid-2012, performing primarily in Germany, as well as in Romania, the Czech Republic, the Netherlands, Austria, and Switzerland.