Nielsen has faced criticism over the a year ago, and has been a subject of recent critical segments from Fox News Channel's Lou Dobbs regarding her actions or, as some see it, perceived inaction on the border crisis.
She added: "This was a firing or quitting of her own making, very few people who seemed to, I would say, embrace the Trump policy of family separation more so than Kirstjen Nielsen".
Minister quits as prospect of ‘softer’ Brexit looms
Brexit Secretary Stephen Barclay said Wednesday that May was entering her talks with Corbyn "without preconditions". Could Britain leave the European Union without agreements, that would be the responsibility of the United Kingdom.
Mr Trump said Kevin McAleenan, the current US Customs and Border Protection Commissioner, will become acting secretary.
Nielsen visited El Paso, Texas, on Wednesday, marking her first stop on a border tour aimed at assessing the surge of migrants and the department's response.
The departure comes just days after Trump suddenly withdrew the nomination of Ron Vitiello for Immigration and Customs Enforcement director, blindsiding the Department of Homeland Security and the Hill.
Multiple airlines experience system-wide outages across U.S.
American Airlines said the technical issue was related to AeroData, a company that makes aircraft location technology. Southwest Airlines was the most heavily affected by the outage as the airline had to delay 620 early flights.
Trump has announced his desire to go in a "tougher" direction.
Nielsen has had a tumultuous tenure that saw Trump steadily ramping up pressure on his team to execute his immigration promises, which he believes is the single driving issue for his base of political supporters.
CBS News reported that it was not clear whether Kirstjen meant to resign voluntarily or made a decision to step down under the pressure.
British PM Theresa May asks European Union to delay Brexit until June 30
If that doesn't work, May plans a series of votes in Parliament to see if a majority-backed strategy can emerge. May is the upcoming elections to the European Parliament, which begin on May 23.
This is a breaking news story and will be updated.