Mr Tillerson savaged Vladimir Putin in a parting statement, adding fuel to the rumours he was fired by Mr Trump because of his hard-line stance on Russia.
Yesterday, two Labour MPs hinted Mr Trump sacked Mr Tillerson because he refused to adopt the official White House stance on Russia.
And in his final speech before returning to private life, Mr Tillerson doubled down on his fiery rhetoric and pointedly refused to thank Mr Trump for his time in the White House.
Mr Tillerson said Russia was displaying “troubling behaviour and actions”, hours after Prime Minister Theresa May said it was “highly likely” the state was behind the assassination attempt on former Russian spy Sergei Skripal.
The now-former US Secretary of State said: “”Russia must assess carefully as to how its actions are in the best interest of the Russian people and of the world more broadly.
“Continuing on their current trajectory is likely to lead to greater isolation on their part, a situation which is not in anyone’s interest.
“Much work remains to respond to the troubling behaviour and actions on the part of the Russian government.
In December Mr Trump said rumours he was preparing to fire Mr Tillerson was “fake news”.
He then tweeted: “He’s not leaving and while we disagree on certain subjects (I call the final shots) we work well together and America is highly respected again.”
However, the two have increasingly diverged on key issues including Russia and North Korea.
Mr Trump, unlike Mr Tillerson, has appeared reluctant to directly condemn Russia for the nerve agent attack on Salisbury.
Yesterday, Labour Co-op MP Mike Grapes said Mr Tillerson’s sacking highlighted the erratic nature of Mr Trump’s presidency.
He said: “It looks as though he used one of his last acts in post to do the right thing and say the right thing about the attempted murders in Salisbury.
“One of the last things he did was make a strong statement of solidarity with the UK. And where is Trump’s statement?
“Normally it takes Trump five minutes to put something out on Twitter – as he did at the time of the London terror attacks – and yet it is more than a week since Salisbury and he has said nothing.
“It is another example of the dysfunction of Trump’s presidency.”
Russia’s envoy to the European Union Vladimir Chizhov said he did not believe Mr Tillerson’s dismissal was due to the Salisbury attack.
He said: “I’m convinced that first, he was fired as the Secretary of State not due to this episode. Second, I have no grounds to expect his successor to take a too mild approach.”