US President Donald Trump ousted Secretary of State Rex Tillerson from his post on Tuesday, replacing him with CIA Director Mike Pompeo.
The 65-year-old learned of his firing after reading Trump's announcement on Twitter, a State Department official revealed.
The move follows a series of public rifts between the pair over issues including North Korea and Russia.
Explaining his decision to reporters at the White House, the president said: "“We were not really thinking the same.
"Really, it was a different mind-set, a different thinking."
On Twitter, Trump thanked Tillerson for his service and said he would be replaced by Pompeo, who would "do a fantastic job!"
Tillerson, who will return to private life, says he will be delegating all of his responsibilities to the State Department's No. 2 official and that he will leave office at the end of the month.
"What is most important is to ensure an orderly and smooth transition during a time that the country continues to face significant policy and national security challenges," he told reporters in a packed briefing room at the State Department.
Pompeo, 54, regularly briefs Trump on intelligence matters, and is considered one of the most hawkish voices on North Korea in the president's inner circle.
A retired army officer, he is a former conservative Republican member of the US House of Representatives from Kansas.
Trump nominated CIA deputy director Gina Haspel to take over as the agency's first female director.
The US President has publicly undercut Tillerson's diplomatic initiatives many times, including this week when the former Secretary of State's comments about Russia appeared at odds with those of the White House.
On Monday, former Exxon Mobil chief executive Tillerson blamed Moscow for the poisonings in England of a former Russian double agent and his daughter. Earlier, White House press secretary Sarah Sanders refrained from saying Russia was responsible.
A senior White House official said Trump asked Tillerson to step down on Friday but did not want to make it public while he was on a trip to Africa. Trump's announcement came only a few hours after Tillerson landed in Washington after a trip that had been cut short.
His ousting marks the biggest shakeup of Trump's Cabinet since he took office more than a year ago.