Nielsen met with Trump at the White House Sunday amid an ongoing influx of migrants at the U.S. -Mexico border that has been taxing America's immigration system and sparking frustration within the administration.
In a subsequent tweet, Trump announced that Kevin McAleenan - commissioner of the U.S. Customs and Border Protection (CBP) - would take over the agency in an acting capacity.
In her resignation letter, Nielsen asked for more from Congress and the courts, which have opposed such Trump administration initiatives as his effort to limit immigration from Muslim nations and the border wall.
Unsurprisingly, White House senior adviser Stephen Miller is reportedly behind the shake-up at DHS. The people say she has been frustrated by difficulty getting other departments to help with the growing number of families coming crossing the border. During the government shutdown over Trump's insistence for funding for a border wall, Nielsen's standing inside the White House even appeared to rise. For one thing, Trump is obsessed with closing the border, which means closing ports of entry where people can cross legally.
Kupcho heads to Augusta National with 1-shot lead
Her time at this first-ever tournament came to an end after the second round, as only the top 30 golfers advanced to the third and final stage.
Nielsen received praise from the president for her handling of the issue, and she stood beside him as he signed the executive order ending the separations.
But in the end she managed to hang on for a little more than a year, having taken up the job in December 2017.
At the same time that Nielsen was being forced to resign, Trump also withdrew the nomination of Ronald Vitiello to head Immigration and Customs Enforcement.
Congress to sue to block Trump border wall funding action
Trump said that he cares more about USA security than about the trade agreement. He has since praised the country for doing more. "We'll tariff their cars".
"President Donald Trump told aides last fall that he wanted to fire Nielsen, and he grew increasingly agitated as a large caravan of Central American migrants reached the U.S. -Mexico border at San Diego". She was heckled in restaurants as the policy came under criticism.
Sources have said Nielsen has been on thin ice before. Among others, Trump now lacks a permanent secretary of defense or chief of staff.
Trump announced the resignation in a tweet today.
Ultimately, acting White House chief of staff Mick Mulvaney seemed to have been able to talk the President out of closing the port of El Paso. Gloria Chavez with the U.S. Border Patrol, center, and Homeland Security Secretary Kirstjen Nielsen listen.
Late show demonstrates Liverpool resilience - Robertson
They also won a critical Champions League group game in the last minute - against Paris St-Germain at the start of their campaign. For all the points we have now, we worked really hard.
Nielsen said the responsibilities of this sprawling, multi-faceted agency - formed in the aftermath of the September 11 terrorist attacks - are "vast and have never been more vital", and that it was "the honor of a lifetime to serve with the courageous men and women" of DHS.