Valerie Harper, who parlayed a sidekick role as the leading lady’s unprepossessing best friend on “The Mary Tyler Moore Show” into a star turn of her own in the hit sitcom “Rhoda,” died on Friday. She was 80.

The death was confirmed by her daughter, Cristina Cacciotti. She did not say where Ms. Harper died.

Ms. Harper’s husband, Tony Cacciotti, announced on Facebook in July that he had decided not to move Ms. Harper into hospice care, as her doctors had recommended. She had leptomeningeal carcinomatosis, in which cancer cells invade the fluid-filled membrane surrounding the brain.

Ms. Harper was a theater actress, working with some regularity but far from well known, when she auditioned for a new CBS sitcom starring Ms. Moore as Mary Richards, a Minneapolis news producer (well, associate news producer) and the embodiment of a newly ascendant American breed — the single working woman.

The part was for her upstairs neighbor, Rhoda Morgenstern, a weight-conscious, self-deprecating, wisecrackingly blunt Jewish expatriate from New York City who would serve as a foil for Ms. Moore’s prim, sweet-tempered, every-hair-in-place and emphatically non-Jewish Mary.