Shannon Guess Richardson, 35, admitted to sending toxic letters to President Barack Obama, New York Mayor Michael Bloomberg and Bloomberg's top gun control advocate Mark Glaze.
In exchange, she accepted a plea deal for a guarantee that she won't serve more than 18 years in prison. The actress was facing life in prison for the charge to which she pleaded guilty.
Richardson was arrested in June after authorities say she mailed three letters from New Boston, located outside of Texarkana, Texas.
Richardson initially told FBI agents her now-estranged husband, Nathan Richardson, had sent the letters, but officials questioned her credibility after interviewing her.
Prosecutors say investigators noted inconsistencies in Richardson's statements and later learned that she had purchased materials online to produce ricin.
John Delk, an attorney who represents her husband, says they believe she was trying to set him up because he had recently filed for divorce.
The letter to Obama, according to a federal indictment, said: "What's in this letter is nothing compared to what ive got in store for you mr president."
Ricin is a potentially fatal poison. It is distilled from the castor plant and converted to powder. It can poison a person through the skin, but is deadliest when it is inhaled.
The letter was intercepted by a White House mail screening facility in late May. The letters to Bloomberg and Glaze were sent around that same time.
The letters did not harm any of the intended recipients, but three emergency responders who examined one of the letters did experience minor symptoms.
Richardson has had minor roles in TV series, including "The Walking Dead and "The Vampire Diaries." The actress has six children including one child born prematurely while she was in custody this year.
A federal judge ultimately will sentence Richardson at a later sentencing hearing, which has not yet been scheduled.
The Associated Press contributed to this report.