Donald Trump has taken to social media to announce that Mark Esper's time as US Defense Secretary has been "terminated".
The First Gulf War veteran has held the post since July 2019, but clashed with the president earlier this year.
In a Twitter thread, Mr Trump announced Mr Esper would be replaced by counterterrorism head Christopher Miller.
In June this year, Mr Esper publicly opposed the threatened use of military troops in response to Black Lives Matter protests in American cities, following the death of George Floyd.
In his tweets President Trump thanked Mr Esper for his service and said Mr Miller will do "a great job".
Mr Miller will take over the role until Joe Biden enters the White House in the new year.
Mark Esper served more than 20 years in the US Army and in the reserve force, having also fought in the First Gulf War.
Presidents who win re-election often replace cabinet members but losing incumbents have previously kept their Pentagon chiefs in place until Inauguration Day to preserve stability in the name of national security.
Professor Michael Clarke, former Director-General at the Royal United Service Institute, told Forces News the development “adds to the sense of chaos and bitterness” in the US.
“I don’t think it will matter in a policy sense because I don’t think much will change in the next 60 or 70 days,” he said, referring to the time between now and Inauguration Day.
The defence expert said Mr Esper had been out of favour for a number of months.
“It doesn’t do President Trump much good because it makes it look as if he is being very personal about this,” he added.
Prof. Clarke went on to claim that the Pentagon had been “fairly disobedient” to its leader in the last year and could resist any moves made by President Trump before he leaves the Oval Office.
Before Mr Esper entered the role last year, it had been vacant for seven months – the longest period ever in which the Pentagon has had no permanent chief.
In late 2018, Jim Mattis resigned from the post amid speculation of a clash with President Trump over troop withdrawal from the Middle East.
Cover image: Mark Esper in 2019 (Picture: PA).