The announcement came after Nielsen met with President Donald Trump, who recently decided, according to the Wall Street Journal, to drop border official Ron Vitiello's nomination to head of U.S. Immigration and Customs Enforcement.
According to the Post, Trump applauded Nielsen over the incident-which was the sort of "tough" action the president wanted from his homeland security secretary.
Yes, welcome to the bleak world in which, despite sanctioning the so-called "zero tolerance" policy that separated more than 2,000 immigrant children from their parents previous year, Nielsen still isn't considered one of the true immigration "hardliners" in the Trump administration, according to CNN.
With such a record, progressive lawmakers like Rep. Pramila Jayapal (D-Wash.) welcomed her departure.
For some Trump critics, this was yet another example of the chaos that supposedly reigns in the White House.
"I did not hear that word used", she told a congressional hearing.
The personnel shake-ups were seen by observers as a signal the president wants to tack an even harder line. This should be a warning to all who believe in justice. Minority Leader Chuck Schumer, "you know he's completely lost touch with the American people". Ms Nielsen was responsible for the proposed border wall between the United States and Mexico, plus the migrants passing through the territory divide.
Making these changes now is an even worse decision, he said, given apprehensions of immigrants at the border are spiking. A source familiar with Nielsen's thinking told CNN Nielsen is taking this as a relief.
Most notably, however, she has become the face of the Trump administration's fierce anti-immigration policy. (The Trump administration has been put on notice by court injunctions ordering the reunification of some families).
Coons added that reports from Nielsen's last meeting with Trump, held on Sunday, noted that she told him what her limits were, and "he didn't accept those limitations and she was sacked".
That wasn't the only apparent roadblock.
Nielsen faced a public outcry over the administration's highly unpopular policy of separating migrant families when they crossed into the U.S.
The Department of Homeland Security is a relatively new federal agency.
A likely force behind such impossible requests was Miller.
The resignation, which was confirmed during a tense meeting at the white house on Sunday, comes after days of speculation that the relationship between the president and the secretary had broken down.