A longtime adviser to Defense Secretary Jim Mattis is resigning, leaving the secretary without one of the first senior officials to join his staff after he was nominated as Pentagon chief.

Sally Donnelly, a former journalist who founded an influential consulting firm before joining the Pentagon, is leaving the Defense Department, Mattis said in a statement Tuesday. He called her a “trusted and valued adviser” for more than a decade and said she played a critical role in the past year in building international partnerships and bringing business reform to the Pentagon.

“I always knew she would return to the private sector and I am grateful for her service,” Mattis said. “I have no doubt that she will stay engaged on the key national security issues the country faces. She departs with my thanks and confidence as she opens the next chapter of her professional career.”

Donnelly quickly joined Mattis as President Trump announced the retired Marine general as his defense secretary nominee in late 2016. She severed ties with the consulting firm she founded, SBD Advisors, to do so.

Donnelly often traveled abroad with Mattis during his first year in office, holding a wide-ranging and sometimes unexplained set of duties that included coordinating meetings with domestic and international groups, nongovernmental organizations and policy analysts. She also previously worked for Mattis as the head of the Washington office of U.S. Central Command, which Mattis commanded from 2010 to 2013.

A senior Pentagon official familiar with the situation said that Mattis and Donnelly always had agreed that she would work for him at the Pentagon for about a year. The official, speaking on the condition of anonymity to discuss personnel matters, said that Donnelly intends to take some time off but that her plans afterward are uncertain. She sold SBD Advisors before joining the government, the official said.

Donnelly declined to comment.

The move leaves the Pentagon without one of its few female senior defense officials. A review of 118 current senior defense officials, including those in uniform, shows 15 others who are women, some of whom are in an acting capacity in the absence of political appointees.

The most senior defense officials include Air Force Gen. Lori Robinson, the chief of U.S. Northern Command and the North American Aerospace Defense Command; Kari A. Bingen, the principal deputy undersecretary of defense for intelligence; Ellen Lord, the undersecretary of defense for acquisition, technology and logistics; and Air Force Secretary Heather Wilson.