McCarthy was born in Haverhill, Massachusetts and grew up in Leawood, Kansas, and graduated from Shawnee Mission East High School.

A widely reported incident in which an intoxicated McCarthy fell down inside a House office building forced her to admit her problem and seek treatment.

McCarthy served as co-chair of the Missouri Commission of Global Climate Change, an extensive two-year (1989–1991) study of scientific data to develop environmental and economic policy options for state action.

In June 2009, her family revealed that McCarthy was suffering from an advanced form of Alzheimer's disease and was residing in a nursing home.

A non-injury car accident involving McCarthy at her home in April prompted her friends to seek medical help which revealed her illnesses.