Lucie Arnaz, daughter of Lucille Ball and Desi Arnaz, confirmed the death in a post on her Facebook page. Singleton died Tuesday, the same day as writer-director Nora Ephron, and Arnaz acknowledged them both in her post, writing, "They were loved and appreciated and will be missed."
Her death also was reported by Variety and the Hollywood Reporter.
Singleton's character, first called Lillian Appleby and then Caroline Appleby, appeared in 10 episodes. She was Lucy's neighbor, the wife of a radio station owner.
"She was sort of Lucy's nemesis" because both had young sons, Singleton said in a 2005 interview for the Archive of American Television of the Academy of Television Arts & Sciences Foundation. "There was a rivalry there but she had to be nice, Lucy had to be nice to Caroline, because her husband owned the radio station."
Harpo Marx was the guest star in one episode.
"Harpo played a concert for us afterward," Singleton said, adding that she loved the episode "I was one of the blondes he was chasing!"
On set, Lucille Ball was nice to Singleton but she was tough when it came to the show.
Actors were replaced "if she didn't like you or you made a mistake or you flopped," Singleton said.
"It was a very difficult show to work, it was challenging," Singleton said, with only a few days of rehearsal and constant script changes.
Born in Brooklyn, Singleton was a professional ballet dancer in her teens, and later became a singer and radio actress.
Singleton also played Margaret Williams on the 1960s sitcom "My Three Sons" and she appeared on dozens of other TV shows in the 1950s, 60s and 70s, including another Lucille Ball program, "Here's Lucy," "Perry Mason," "All in the Family," and "Days of Our Lives."
She was married to comedy writer Charlie Isaacs for 61 years before his death in 2002.