Her testimony before a Senate Judiciary subcommittee will likely center on ousted national security adviser Michael Flynn and his ties to Russia.
Here are three things you need to know.
1. President Trump fired her in January
An Obama appointee, Yates was fired from her position as acting attorney general in January, when she said she would refuse to defend the Trump Administration's travel ban in court.
"I am responsible for ensuring that the positions we take in court remain consistent with this institution's solemn obligation to always seek justice and stand for what is right," Yates wrote in a letter at the time.
White House press secretary Sean Spicer said she "betrayed the Department of Justice by refusing to enforce a legal order designed to protect the citizens of the United States." Yates' appearance on Monday will be her first testimony before Congress since she was fired.
2. Her scheduled testimony was canceled before
Yates was supposed to testify before the House Intelligence Committee in March, but the hearing was abruptly canceled without explanation by Republican chairman Devin Nunes. Reports surfaced at the time that the White House sought to block Yates from testifying because her testimony would cover topics "likely covered by the presidential communications privilege and possibly the deliberative process privilege."
But the White House denied the claim. "The White House has taken no action to prevent Sally Yates from testifying, and the Department of Justice specifically told her that it would not stop her, and to suggest otherwise is completely irresponsible," the Administration said in a statement.
She's been invited back to testify.
3. She will testify that she warned the White House
On Monday, she's expected to testify that she warned White House officials about then-national security adviser Michael Flynn's contact with Russian Ambassador Sergey Kislyak, which would contradict the Administration's characterization of what she told them.
According to the Associated Press, Yates will say that she told the White House counsel's office that she believed Flynn's misleading statements on his contact with Kislyak would leave him compromised, CNN reports. (Flynn was removed from his post in February over questions about his contact with Kislyak.)
The White House, however, has said Yates merely gave them a "heads-up" about Flynn's Russia contacts. Yates' testimony will be her first public account of that conversation with the White House.