Mintz, however, denied reports the 83-year-old artist had suffered a stroke, reported BBC.
He said she went to hospital on the advice of her doctor and was expected to be discharged and return home later on Saturday.
Her son Sean Ono Lennon tweeted that his mother was doing well and cause of hospitalisation was dehydration/tiredness not stroke.
Ono was married to former Beatle John Lennon until he was murdered in 1980 outside New York's Dakota Building, where she still lives.