Though President Trump and McGahn had previously discussed when the counsel might leave the White House, McGahn reportedly was not given a head's up that the news was coming.
Trump made the announcement on Twitter on Wednesday, ending widespread speculation about McGahn, who will be the latest in a long string of high-ranking advisers to leave Trump's side.
Along with other current and former Trump aides, McGahn has met on multiple occasions with Mueller's team, for dozens of hours.
"I hope it's not true McGahn is leaving WhiteHouse Counsel".
"Senator Grassley has admired Mr. McGahn's work over the last two years and has considered him integral to the President's record-breaking success on filling judicial vacancies", the spokesman said.
Axios reported McGahn wants Emmet Flood to replace him. We also learned that Trump himself was unaware that McGahn had spent so much time with Mueller's team, and that the White House more broadly was unaware of what exactly McGahn had told Mueller.
Emmet Flood, who now directs the Russian Federation legal strategy from inside the White House, is a potential replacement, CNN reported last week.
Minutes after Grassley's warning, however, the president fired off another tweet, again referring to Mueller's probe as a "witch hunt".
Trump named McGahn White House counsel in November 2016 shortly after winning the presidential election.
The president later denounced the reports as "fake news".
Flynn was forced to resign after White House officials concluded he had misled them about the nature of his contacts with Kislyak during the White House transition.
The New Jersey native had been a partner at Jones Day, one of the world's largest law firms.