McKinley declined to comment on his testimony.
A former senior State Department official told CNN that McKinley was "known as a man of integrity, a man of principle".
"If he had encountered something that happened either in the State Department, or if he felt that he could no longer fulfill his duties without jeopardizing his integrity and principles, he would be someone who felt to have no choice but to resign. "They said.
McKinley spent more than three decades on foreign service and served as ambassador to Peru, Colombia, Afghanistan and Brazil.
In a farewell email to US Department of State counterparts received by CNN on Friday, McKinley wrote that he "leaves the department to pursue other opportunities where they might lead".
"The decision is personal: it's time after 37 years in the department," he wrote.
Asked about McKinley's departure on Friday in an interview in Nashville, Pompeo said his counselor wanted to "begin the next phase of his life."
I've known Mike McKinley for some time, and he's been in the State Department for 37 years. He told me for many good and sufficient reasons for him and his family to continue and move on to the next phase of his life, "he told WKRN-TV, a CNN subsidiary.
Pompeo introduced McKinley as a consultant and liaison The career diplomat was a trusted advisor and frequently traveled with Pompeo on business trips.
CNN's Manu Raju, Jeremy Herb, Jamie Gangel and Caroline Kelly contributed to this report.