Capshaw was born in 1953 Kathleen Sue Nail in Fort Worth, Texas and moved to St. Louis, Missouri at the age of five. She graduated from Hazelwood Senior High, now called Hazelwood Central High School in 1972. She earned a degree in education from the University of Missouri–Columbia, where she was a member of Alpha Delta Pi. She taught Special Education at Southern Boone County High School in Ashland, Missouri and Rock Bridge High School in Columbia, Missouri, but later moved to New York to pursue her dream of acting, landing her first role on the soap opera Love of Life. She took the surname Capshaw upon her marriage to Robert Capshaw on January 1976; they had one child, actress Jessica Capshaw, and eventually divorced in 1980.