A senior officer who has held a range of diplomatic posts, including ambassador to Afghanistan, Colombia and Peru, McKinley was serving as ambassador to Brazil past year when Pompeo recruited him as a policy adviser and a conduit between his office and the career service.
Secretary of State Mike Pompeo spoke with Gray TV's Greta Van Susteren this week amid Turkey's invasion of Syria, the arrests of Rudy Giuliani's associates and an impeachment inquiry on the president.
The resignation comes as Pompeo faces mounting stress from Home investigators after it emerged that he was on the road when President Trump requested Ukrainian President Volodymyr Zelensky to probe former Vice President Joe Biden and his son Hunter.
While McKinley has not been directly involved with Ukraine, he has served as a conduit between Pompeo's office and career staff, according to the Washington Post, which first reported his resignation Thursday night.
US Secretary of State Mike Pompeo has dismissed reports that his aide chose to leave because the former was not doing enough to protect employees called to testify about President Trump's July call to Ukrainian President Volodymyr Zelensky, which subsequently kick started an impeachment inquiry.
Asked about McKinley's departure in an interview in Nashville on Friday, Pompeo said his adviser "wanted to go on and begin the next phase of his life".
McKinley was involved in several policy spheres, holding key roles in foreign policy efforts in Venezuela, Mexico and Afghanistan, the Post reported - the last a key worldwide focus after Trump revealed last month that Taliban leaders had been slated to travel to the United States for secret peace talks but that the meeting was canceled as he called off peace talks with the militant group entirely.
The Scream Department earlier this week blocked testimony by the United States ambassador to the European Union, Gordon Sondland, who used to be then subpoenaed to appear October 16.
"I protect every single State Department employee", Pompeo said. "When the State Department employees are doing things right, when they're behaving in ways that are consistent with the rule of law and working under President Trump's mission, I'll always stand for them".