Pascale Ferrier, 55, presented her pleadings in US District Court in Washington on Wednesday.
She also pleaded guilty to eight counts relating to similar bioweapons offenses committed in Texas.
Judge Dabney L. Friedrich has yet to approve the 21-year, 10-month sentence recommendation with a hearing scheduled for April 26.
Ms Ferrier was arrested at a US border crossing on September 20, 2020 and accused of sending Mr Trump a letter containing the poisonous poison ricin, which was intercepted on September 18 before it was delivered to the White House.
According to US authorities, Ms Ferrier also sent letters containing ricin to law enforcement and correctional officials in Texas linked to her arrest and detention there in the spring of 2019.
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The U.S. Department of Justice said in a statement that court documents show Ms Ferrier admitted to making deadly ricin at her home south of Montreal in September 2020.
There is no place for political violence in our country and no excuse for threatening our officials or endangering our public servants, US Attorney for the District of Columbia Matthew M. Graves said in a statement. We hope this decision will serve as a warning that using our mail system to send toxic substances and other such threats will cost you your freedom for many years to come.
Prosecutors say the letter to Donald Trump contained similar language to letters sent to Texas officials. The letter to Mr Trump called the poison a special gift and ended with a threat to find a better recipe for a different poison, or the letter writer threatened to use her gun if she could come. The letters were all signed Free Rebel Spirit [Esprit rebelle libre, NDLR].
In an affidavit to obtain a warrant, a federal police investigator, the FBI, wrote that the envelope contained a powdered white substance with a letter addressed to Mr. Trump, calling him the cheeky clown bully. The letter, intercepted less than two months before the 2020 presidential election, accused Mr Trump of bankrupting the United States and urged him to abandon his bid for re-election.
Authorities said when Ms Ferrier was arrested she was in possession of a handgun and several other weapons, nearly 300 rounds of ammunition and a fake Texas driver’s license.