Sally Yates had been in the crossfire over her constitutional duty before her ouster Monday night as acting attorney general.
An Obama appointee, Yates had been running the Justice Department while Sessions underwent confirmation as attorney general in the Senate.
She was fired Monday night after instructing Justice Department lawyers not to make legal arguments defending Trump's executive order on immigration and refugees.
"My responsibility is to ensure that the position of the Department of Justice is not only legally defensible, but is informed by our best view of what the law is after consideration of all the facts," Yates said in a letter to department lawyers.
The longtime Justice Department official paid the price for falling afoul of the Trump White House.
"(Yates) has betrayed the Department of Justice by refusing to enforce a legal order designed to protect the citizens of the United States," a White House statement said.
Yates' replacement, Dana Boente, the US attorney for the Eastern District of Virginia, was sworn in at 9 p.m. ET Monday, according to an administration official.
A few hours later, Boente issued a statement rescinding Yates' order, instructing Justice Department lawyers to "defend the lawful orders of our President."
The Senate Judiciary Committee is voting Tuesday on the nomination of Sessions as attorney general. It's expected to approve and advance him for a vote in the full Senate.