Simon, 83, shared the diagnosis in a statement to ABC News on Monday, explaining that she has spent time learning to live with the progressive neurological disorder and deciding whether to discuss it publicly.
"So many people have written to me, kindly wondering about my relative silence, asking how I am and what I have been doing. The truth is, I've been learning how to live with Parkinson's disease," Simon said.
"It has taken me some time to understand the diagnosis, to adjust to it, and to decide how much I wanted to say about it publicly," she continued. "Parkinson's is different for everyone, and it can be unpredictable. Some days I'm so tired I can't get the day moving at all. On others, it gives me a little more room to move, think, work, and feel like myself."
The "You're So Vain" singer said her health problems began with arthritis in both knees and one hip. She eventually underwent three joint replacement surgeries and initially believed her difficulty walking was part of the recovery process.
"But my mobility continued to worsen. I had trouble standing up from low chairs and deep couches without someone offering me an arm," Simon said.
Simon said there were eventually periods when she could not walk without "considerable help."
"My family and I knew that something more was going on. After an extensive evaluation at the Mayo Clinic, I was diagnosed with Parkinson's," she said.
Simon said she began treatment, including medication to help with stiffness and other symptoms.
"There is no tidy or predictable schedule to the illness. It does not consult my calendar before deciding what kind of day I am going to have," she said.
Parkinson's disease is a progressive neurological disorder that primarily affects movement. An estimated 1.1 million people in the U.S. are living with Parkinson's, and nearly 90,000 people are diagnosed each year, according to the Parkinson's Foundation.
Common movement symptoms include tremors, stiffness, slowed movement and balance problems, according to the National Institute of Neurological Disorders and Stroke. Parkinson's can also cause non-movement symptoms, including depression, anxiety, apathy, sleep problems and cognitive changes.
Simon said apathy has been particularly difficult for her to explain.
"It is not simply sadness or laziness. It is as though the part of the brain that sends out invitations to participate in life has temporarily misplaced the guest list," she said.
There is currently no cure for Parkinson's, but treatments can help manage symptoms and improve quality of life.
Simon also revealed she was treated for basal cell carcinoma on her face during the same period. She said the cancer was removed, but the surgery affected her appearance and made her more self-conscious about being seen publicly.
"Between my mobility issues, the Parkinson's diagnosis, the surgery, and the emotional effects of it all, withdrawing from public view was the most palatable reaction," Simon said.
Despite those health challenges, Simon said she has continued working, including recording a new album, "Comes in Waves," which is scheduled for release Aug. 14.
"Working on the music gave shape to days that did not always have much shape. It gave me somewhere to go without having to leave the room. It reminded me that illness can change your life without becoming the whole of your life," she said.
Simon, whose career spans more than five decades, has won two Grammy Awards and an Academy Award for "Let the River Run," from the 1988 film "Working Girl."
"I do not consider Parkinson's a gift or a blessing. It is neither. It is difficult, frustrating, and sometimes frightening," Simon said. "I am still learning how to live with it and how to accept it without feeling that I have surrendered something essential."
Simon concluded her statement by thanking her children, family, friends, caregivers and medical professionals for their support.
"These days I move more slowly, I lean on others more than I once did, and I have learned to accept that every day will look a little different. But I am still very much here," she said.