Borca studied music at the University of Wisconsin with John Barrows and Arthur Weisberg, graduating in 1971. While there she met Cecil Taylor, who taught there during the 1970/71 academic year; she studied with him and played in his big bands, ensembles and the Cecil Taylor Unit, and was his assistant at Antioch College, Ohio, in the Black Music Program. She was an assistant to saxophonist Jimmy Lyons in 1974 during his artist in residence at Bennington College, Vermont and married the longtime Taylor sideman; she played in Lyons's band for 12 years until he died in 1986. She performed with her own bands at the Newport Jazz Festival New York City Salute to Women in Jazz, '78 and '79, Soundscape, Vision Festival, Jazz Fest Berlin and other festivals, concerts, clubs and lofts.
She has performed with her own bands in New York City, the U.S. ,and Europe, with musicians such as Cecil Taylor, William Parker, Bill Dixon, Butch Morris, Marco Eneidi, Joel Futterman, Sonny Simmons, Alan Silva, and Jackson Krall.
With Jimmy Lyons
- Riffs (Hat Hut, 1980)
- Give It Up, Jimmy Lyons (Black Saint, 1985)
- Wee Sneezawee (Black Saint, 1983)
- The Box Set (Ayler, 1984, 1985)
With Alan Silva
- Red Snapper, Paul Murphy (Cadence Jazz 1982)
- Moments, Joel Futterman (Ear Rational, 1983)
- Passage, Joel Futterman (Ear Rational, 1984)
- Winged Serpent (Sliding Quadrants), Cecil Taylor (Soul Note, 1986)
- Many Rings, Joe Morris (Knitting Factory, 1999)
- Final Disconnect Notice, Marco Eneidi (Botticelli, 1994)
- Waking the Living, Earth People (Undivided Vision, 2001)
- The Cosmosamatics, Sonny Simmons (Boxholder, 2001)
- 17 Musicians in Search of a Sound: Darfur, Bill Dixon (AUM Fidelity, 2008)
- "Karen Borca | Credits". AllMusic. Retrieved 3 January 2017.