The England and Wales Cricket Board (ECB) announced on Sunday the New Zealander would stand down as the country's Test coach with immediate effect, though he'll continue to lead the England men's white-ball teams.
McCullum said that he was "gutted" his four years as coach had been brought to an end, but that he respected the ECB's decision.
Coming only two weeks after captain Ben Stokes's shock retirement, McCullum's sacking appears to close the final chapter on the controversial Bazball experiment, in which England eschewed a traditional Test match approach for audaciously aggressive, many would say reckless, strategies.
England under McCullum recorded 27 wins, 20 losses and two draws, with seven of those losses coming in England's last nine Test matches.
That's when criticism of Bazball in England rose to a crescendo, particularly after their stinging 4-1 loss in Australia last summer.
In a statement released by the ECB, McCullum said he had "loved coaching the Test side" and was "incredibly proud of what we've achieved together".
"There've been some unbelievable highs and a few tough days along the way, but that's all part of taking on a challenge like this," the former New Zealand wicketkeeper-batsman said.
"It's been a privilege and an honour, and I'm grateful. Grateful to the players, the staff and the fans who supported us on the journey.
"Of course, I'm gutted not to be continuing, but I respect the decision. My focus now is on giving everything I've got to the white ball teams and helping England keep moving forward.
"I wish the Test team nothing but success. There's a hell of a lot of talent in that dressing room and they're a special bunch of lads.
"I'll always be backing the boys, with a smile on my face, and hoping they keep taking the game on. I know they'll continue to make people proud."
ECB chief executive Richard Gould said McCullum had "breathed new life" into England's Test team, bringing some "amazing victories", and that the Board was "grateful for all he has given to the role".
"We now believe that the time is right to make a change for the Test team as we target victory in The Ashes next summer," Gould said.
England men's managing director Rob Key said some of England's most memorable moments in recent history had occurred during McCullum's tenure.
"It's been an absolute privilege to watch him shape the mentality of the team, to one the players have loved, and see him develop a new generation of talent who will be at the heart of England men's teams for years to come," Key said.
"He leaves the Test team well-set and poised to achieve great things."
Noting McCullum's England T20 team had recently become the No.1 ranked side in the world, the ECB said the search for a new Test coach would now begin.
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