The top US diplomat for Europe resigned Tuesday, citing personal reasons, opening a new vacancy at time of uneasiness among traditional US allies with President Donald Trump's approach.
"As the administration starts its third year, I feel I have completed what I set out to do -- develop the department's Europe strategy," Mitchell said in a statement.
Unlike recent high-profile resignations over Trump's pullout from Syria, Mitchell did not indicate he was protesting any policy.
Secretary of State Mike Pompeo wrote on Twitter that Mitchell had "done an outstanding job" and wished him "much happiness" with his family.
His departure comes after France, Germany and, to a lesser extent, Britain voiced unease over Trump's sudden withdrawal from Syria, where 2,000 US troops have been fighting the Islamic State movement and maintaining an unstated truce with Turkey.
Mitchell's tenure was dogged by unusually pointed exchanges between European allies and Trump, who has cast doubt on the value of the NATO alliance and broken the diplomatic taboo on criticizing friendly nations.
German Chancellor Angela Merkel has made no secret of her disdain for Trump, who has repeatedly attacked her welcoming of more than one million asylum seekers, while Trump has ridiculed French President Emmanuel Macron's domestic standing after he criticized the US leader's nationalist ideology.
Mitchell's portfolio also covers Russia, which has sought a warm relationship with Trump personally even as the United States keeps building pressure over issues including Moscow's support of separatists in Ukraine and its alleged meddling in US elections.
A German speaker who holds a doctorate in political science from the Free University of Berlin, Mitchell is a veteran Washington think-tank expert and co-founded the Center for European Policy Analysis.
Mitchell, who advised Mitt Romney during his failed 2012 president campaign, has advocated a traditional Republican foreign policy that emphasizes a strong US military edge against potential rivals.
Mitchell was confirmed by the Senate in October 2017 after an uncharacteristically long gap from the previous administration as the Trump administration vetted nominees for loyalty.
Elisabeth Millard, a career diplomat, will succeed Mitchell in acting capacity until the administration proposes a replacement.
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