Variety reports that the actress -- who played Lucy and Desi's meddling, hyper-competitive neighbor Carolyn Appleby -- passed away Tuesday at the age of 92 in Los Angeles.
Over the course of her three-decades long career in TV, Singleton guest starred on shows like The Munsters, Dick Van Dyke, Gomer Pyle, U.S.M.C., Gunsmoke, and Twilight Zone.
Lucille Ball and Desi Arnaz's daughter Lucie Arnaz took to her Facebook page earlier this week to eulogize her mother's longtime friend and colleague.
"A day of saying hasta luego to two great ladies, Nora Ephron and Doris Singleton," she wrote (Singleton passed away on the same day as the legendary screenwriter). "May they both fly swiftly heavenward and enjoy a blissful rest for jobs well done down here. They were loved and appreciated and will be missed."
Singleton's spouse of over 60 years, veteran TV writer and producer Charles Isaacs, passed away in 2002.