She was born in Hollywood, California and lived there and in Pismo Beach most of her life. Before becoming a professional writer she spent many years in theater, including teaching Elizabethan English as a second language.
She is best known for her "Company" series of historical time travel science fiction. Her first stories were published in Asimov's Science Fiction in 1997, and her first novel, In the Garden of Iden, by Hodder & Stoughton in the same year. Other notable works include Mendoza in Hollywood (novel, 2000) and "The Empress of Mars" (novella, 2003), which won the Theodore Sturgeon Award and was nominated for a Hugo Award.
Her unusual first name (pronounced like the word "cage") is a combination of the names of her two grandmothers, Kate and Genevieve.
In 2008, she donated her archive to the department of Rare Books and Special Collections at Northern Illinois University.
In 2009, her short story "Caverns of Mystery" and her novel House of the Stag were both nominated for World Fantasy Awards, but neither piece won.
In January 2010, it was reported that Baker was seriously ill with cancer. She died from uterine cancer at approximately 1:00 a.m. on January 31, 2010 in Pismo Beach, California.
In 2010, Baker's The Women of Nell Gwynne's was nominated for a Hugo Award and a World Fantasy Award in the Best Novella categories. On May 15, 2010, that work was awarded the 2009 Nebula Award in the Best Novella category.