The plea stemmed from her July 13 car crash after which police said she failed multiple sobriety tests and slurred her speech.
TMZ quoted her as saying in court Tuesday she hadn't been on drugs when she drove her car into a tractor-trailer, but rather suffered a seizure as a result of an old head injury.
She also admitted in court she may have told police after the accident she had taken an Ambien sleeping pill before the collision, but now believes she was mistaken and tests showed "no alcohol, recreational drugs or prescription medication in my system," TMZ reported.
Kennedy said she underwent an MRI after the crash and the test revealed "an area of hyper density" in her brain, which she said is related to an old head injury.
"This accident was caused not by a sleeping aid but by a complex partial seizure," Kennedy said.
She is due back in court next month.
Kennedy, 52, is the daughter of the late U.S. Sen. Robert F. Kennedy and Ethel Skakel Kennedy. She also is the ex-wife of New York Gov. Andrew Cuomo and mother of his three children.
Notable deaths of 2014 [PHOTOS]
Mindy Kaling mistaken for Malala Yousafzai