Gladys Horton, who gathered some of her high school friends into a singing group that became the Marvelettes and then sang lead vocals on “Please Mr. Postman,” which became Motown Records’ first No. 1 hit, died on Wednesday in Sherman Oaks, Calif.

Her son Vaughn Thornton said in an interview that her health had been in decline for several years. In a statement released by the Los Angeles chapter of the Motown Alumni Association (an independent group not associated with Motown Records, which is now an affiliate of Universal Music), he said she had not recovered after suffering a stroke.

Ms. Horton was in her mid-60s, but her precise age was uncertain. The statement said she was born in 1944, but Mr. Thornton gave his mother’s birthday as May 30, 1945, making her 65 at her death.

Ms. Horton was in a high school glee club in Inkster, Mich., outside Detroit, when she recruited three of her classmates  Katherine Anderson (now Schaffner), Georgeanna Tillman and Juanita Cowart  as well as a friend who had recently graduated, Georgia Dobbins, and formed a quintet. They called themselves the Casinyets  a contraction of the words “can’t sing yet,” an acknowledgment of their lack of experience.