Only 11 days ago, McGurk had said it would be "reckless" to consider Daesh defeated and therefore would be unwise to bring US forces home.
He decided to speed up his plan to leave in mid-February, AP reported.
McGurk, appointed to the post by President Barack Obama in 2015 and retained by Trump, said in his resignation letter that the Takfiri militants were on the run, and that the premature pullout of US forces from Syria would create the conditions that gave rise to Daesh.
McGurk also cited gains in accelerating the campaign against Daesh, but that the work was not yet done.
McGurk, whose resignation is effective Dec. 31, was planning to leave the job in mid-February after a US-hosted meeting of foreign ministers from the coalition countries, but he felt he could continue no longer after Trump's decision to withdraw from Syria and Mattis' resignation, the report said.
In response, Trump took to the Twitter and said, “Brett McGurk, who I do not know, was appointed by President Obama in 2015. Was supposed to leave in February but he just resigned prior to leaving. Grandstander? The Fake News is making such a big deal about this nothing event!”