The State Department's third-ranking official and top diplomat is stepping down from his post.

Undersecretary of State for Political Affairs Thomas Shannon told The Associated Press that he has decided to leave the department for personal reasons after nearly 35 years in the U.S. Foreign Service.

"I decided it was an important moment to take a step back and to determine what next I can do in my life," he said.

Heather Nauert, a spokeswoman for the State Department, said in a tweet on Thursday that Shannon will stay on until his successor is named.

Undersecretary Shannon has given a lifetime of service to the American people @StateDept. He’s a patriot, a diplomat and a great American.

Working with him has been one of the great honors of my life. He will stay on until a successor is named to ensure a smooth transition. — Heather Nauert (@statedeptspox) February 1, 2018

Shannon, who was confirmed to his current undersecretary position in 2016, was one of the most senior State Department officials to remain in his job after President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE took office last year. He has served under six presidents, and is widely respected in the U.S. diplomatic community.

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Shannon's retirement comes as Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE faces criticism for driving out career diplomats. The U.S. ambassador to Panama, John Feeley, resigned in January, saying that he felt he could no longer serve under the Trump administration.

But Shannon told the AP that his decision was largely driven by the fact that he recently turned 60, as well as by the death of his mother late last year. He said that he stayed on to help the department navigate the transition between administrations.

"I’ve served well across 35 years," Shannon said. "I’ve fought the good fight. I’ve lived by my oath and I want to go out living by my oath, which is respect for the Constitution, respecting our political institutions, respecting our values and respecting the choices that the American people have made."

Tillerson praised the undersecretary as the "senior statesman," and told the AP that Shannon would be welcome to return to the State Department if he ever wanted to.

"There will always be a place for Tom Shannon at the State Department," Tillerson said.

Tillerson in a later statement congratulated Shannon on his career, saying his time was “well spent.”

“Tom’s counsel has been well received over multiple administrations. I particularly appreciate his depth of knowledge, the role he played during the transition ... and his contributions to our strategy process over the past year,” Tillerson said.

“Tom has served his country at home and abroad. At each post, he has also served his colleagues. Tom exhibits a devotion to service that has and shall continue to inspire State Department colleagues.”

--Morgan Chalfant contributed to this report, which was updated at 1:37 p.m.