President Donald Trump on Thursday fired his embattled Homeland Security Secretary Kristi Noem, after mounting criticism over her leadership of the department, including the handling of the administration's immigration crackdown and disaster response.Trump, who said he would nominate in her place Oklahoma Republican Sen. Markwayne Mullin, made the announcement on social media on Thursday, two days after Noem faced a grilling on Capitol Hill from GOP members as well as Democrats.Trump says he'll make Noem a "Special Envoy for The Shield of the Americas," a new security initiative that he said would focus on the Western Hemisphere.Noem, took the stage to address a Department of Homeland Security event moments after Trump's announcement but made no immediate mention of her ouster. Instead, she read from prepared remarks, including reinforcing Trump's message from the State of the Union last month.Noem is the first Cabinet secretary to leave during Trump's second term. Noem's departure caps a tumultuous tenure overseeing immigration enforcement tactics that have been met with protests and lawsuits.Noem's tenure looked increasingly short-lived after hearings in Congress this week where she faced rare but blistering criticism from Republican lawmakers. One particular point of scrutiny was a $220 million ad campaign featuring Noem that encouraged people in the country illegally to leave voluntarily. Noem told lawmakers that Trump was aware of the campaign in advance, but Trump disputed that in an interview Thursday with Reuters, saying he did not sign off on the ad campaign.Her department, DHS, has also been shut down for 20 days, although many of the employees are continuing to work, often without pay.Noem has faced waves of criticism as she's overseen Trump's immigration crackdown, especially since the shooting deaths of two protesters in Minneapolis at the hands of immigration enforcement officers. The former South Dakota governor was also criticized over the way her department has spent billions of dollars allocated to it by Congress.Frustrations over Noem's execution of the Republican president's hard-line immigration agenda — particularly her leadership after the shooting deaths — as well as her handling of disaster response, paved the way for her downfall. She faced blistering criticism from Democrats, and some Republicans, in the Congress hearings this week.Aside from immigration, Noem also faced criticism — including from Republicans — over the pace of emergency funding approved through the Federal Emergency Management Agency and for the Trump administration's response to disasters.Mullin would need to be confirmed by the Senate, but under a federal law governing executive branch vacancies, he would be allowed to serve as an acting Homeland Security secretary as long as his nomination is formally pending.