Warner was born in Manhattan on February 9, 1965. Her mother, Naomi (née Bernstein), is a literary agent, an independent marketing consultant, and a licensing director. Her father, the late Neil Warner, was a jingle composer, a pianist, an arranger, and a former jazz bandleader. Her paternal grandfather was Hollywood composer Jack Shilkret, the brother of composer Nathaniel Shilkret. Warner attended the Dalton School at age twelve. There she met an agent who advised Warner to consider acting. Shortly thereafter, Warner landed a role on the soap opera Guiding Light. Warner earned a degree in theater arts from Brown University in 1987. After her graduation, Warner moved to Los Angeles, where she worked as a waitress while auditioning for acting.