Bynes, 26, also was charged with drunken driving and driving with a suspended license.
People.com said the "What a Girl Wants" and "Hairspray" star was not in court for Thursday's plea hearing, which pertained to incidents on April 10 and Aug. 4.
Her next court hearing is Oct. 19, People.com said.
N.J. man wakes up from 10-hour sleep with knife in back
Campus cop fatally shoots Texas student during traffic stop