Mariah Carey, the superstar singer who has lived in the public eye for three decades, has acknowledged that, in 2001, she learned that she had bipolar disorder.

Ms. Carey disclosed the diagnosis in an interview with People magazine’s editor in chief, Jess Cagle. A preview of the magazine’s cover story was published online Wednesday. The full interview will be available Friday.

The interview marks one of the first instances in which a celebrity of Ms. Carey’s stature has acknowledged her struggles with mental illness. In the interview, she explained why she had not previously revealed the diagnosis.

“I didn’t want to carry around the stigma of a lifelong disease that would define me and potentially end my career,” she said. “I was so terrified of losing everything.”