A top aide to Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE is leaving the Pentagon to return to the private sector, Mattis announced Tuesday.

In a statement, Mattis said that senior adviser Sally Donnelly is leaving with his “thanks and confidence as she opens the next chapter of her professional career.” He "always knew she would return to the private sector," he added.

“For over a decade, Sally Donnelly has been my trusted and valued adviser,” Mattis said in the rare public good-bye statement. “She has played a critical role in the Department of Defense throughout this important first year. She was particularly effective in shaping two of the department’s major lines of effort: building international partnerships and bringing essential business reform to the Pentagon.”

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Donnelly has served as Mattis’s senior adviser since he was sworn in on Jan. 21, 2017, and immediately prior, guiding him through the confirmation process.

She also worked with him as the head of the Washington office for U.S. Central Command (Centcom) when Mattis was the general in charge of Centcom.

Prior to that assignment, Donnelly was the special assistant to now-retired Adm. Mike Mullen when he was the chairman of the Joint Chiefs of Staff.

Before joining Mattis back at the Pentagon last year, she founded and managed SBD Advisors, a consulting firm, in which she advised “technology and corporate clients as well as non-governmental organizations on strategic positioning, communications and policy issues,” according to her Pentagon bio.

In his statement, Mattis said he is "grateful for her service."

“I have no doubt that she will stay engaged on the key national security issues the country faces," he said.