Simon, 83, explained her absence from the spotlight has been due to her ongoing health issues.
"So many people have written to me, kindly wondering about my relative silence, asking how I am and what I have been doing," she said in a statement to the Associated Press.
"The truth is, I've been learning how to live with Parkinson's disease.
"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."
Parkinson's disease is a neurodegenerative condition in which parts of the brain become progressively damaged over the years.
The main symptoms include involuntary shaking, stiff and inflexible muscles and slow movement. There is currently no cure.
Simon said there have been times when she has been unable to walk without help.
"Parkinson's is different for everyone, and it can be unpredictable.
"The problems began with arthritis in both knees and one hip. I eventually had all three joints replaced, out with the old and in with delicate bouquets of metal and plastic.
"After three replacement surgeries, I assumed my difficulty walking was simply an unfortunate and rather ironic part of the recovery process. But my mobility continued to worsen.
"After an extensive evaluation at the Mayo Clinic, I was diagnosed with Parkinson's.
"Parkinson's is usually associated with movement, tremors, and balance, but it can affect much more than the body. It can bring anxiety, depression, exhaustion, and apathy.
"You can find yourself lying there like a starfish drying in the sun, arms pointing in all directions, while nothing inside is telling you to get up, read, watch, write, sing, call someone, or do much of anything at all.
"But I am still very much here. With love, Carly."
Amid her Parkinson's diagnosis, Simon revealed she has also been receiving treatment for basal cell carcinoma, the most common type of skin cancer.
She had surgery to remove the cancer, but the impact on her appearance made her reluctant to be seen in public.
Simon rose to fame in the 1970s with a string of hit singles including Anticipation, You Belong To Me, Nobody Does It Better and You're So Vain.
Comes In Waves, Simon's first album in almost 15 years, will be released on August 14.
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