Ms. Flack was performing a charity concert for the Jazz Foundation of America when she fell ill.
New York City Fire Department (FDNY) said the ambulance arrived around 8:50 PM EST, and she was taken to a hospital in Harlem.
The 81-year-old had enormous hits in the 70s and 80s such as "Killing Me Softly," "The First Time Ever I Saw Your Face" and "Set the Night to Music."
It is believed the singer was able to leave the stage on her own.
Musician James Mtume had shared a photo of himself with Ms. Flack on Instagram shortly before the incident took place.
He wrote: “Back together again! With the best!”
Despite one of her most famous songs being "Killing Me Softly”, the 81-year-old was not the first artist to sing the track.
Lori Lieberman first recorded the song back in 1971.
Ms. Flack’s version of the song released two years later.
The artist is currently the only singer to have won the Grammy Award for Record of the Year two years in a row.
In 1999 the sensation was awarded a star on the Hollywood Walk of Fame.
Nelson Mandela attended a concert of hers in South Africa the same year.
The singer was born on February 10, 1937.