Shannon Guess Richardson entered the plea during a federal court appearance in Texarkana. Her guilty plea comes under a deal that caps any prison sentence at 18 years.
Her attorney, Tonda Curry, said Richardson was ready to admit her role in sending toxic letters to Obama, Bloomberg and the head of Bloomberg's pro-gun control group.
Prosecutors say that before her arrest in June, Richardson tried to frame her now-estranged husband for mailing the letters containing ricin, a powdery substance that can cause respiratory failure if inhaled.
Richardson has had minor roles in the television series "The Walking Dead" and the movie "The Blind Side."
