Austin said he was “deeply grateful for her tremendous service,” and that she was “instrumental” in navigating difficult challenges within the Pentagon and internationally over her years in the job.

“From day one of this administration, Kelly’s leadership, counsel, and selfless service made our nation safer, made the lives of our people better and more rewarding, and rendered the heavy burden of this office of mine a good bit lighter. At every stage, she provided a steady hand guiding our staff and the Department. I am – and will remain – in her debt,” Austin said in a statement to POLITICO announcing her departure. “If there is a better example of exemplary public service at a challenging time, I do not know of it.”

Magsamen is described as a key interlocutor between the Pentagon and the White House and a trusted adviser to Austin. She had close connections in the White House, having served on the National Security Council in the George W. Bush and Barack Obama administrations, and on the NSC’s transition team during the Biden-Trump transition.

Austin could always rely on Magsamen to “pick up the phone and call Jake Sullivan or Jon Finer” on a regular basis, said Colin Kahl, a former undersecretary of defense for policy, referring to the national security adviser and his deputy.

In a statement to POLITICO, Sullivan called Magsamen a “force multiplier for this administration and for the entirety of our U.S. national security enterprise.”

“Not only was Kelly a trusted advisor to a historic secretary of defense, but she also became an indispensable teammate across the interagency, helping guide our nation’s largest department through numerous national security challenges and keeping us all safe,” Sullivan said. “She has left an indelible mark on this administration and we will miss working with her.”

Magsamen, who was the vice president for national security and international policy at the Center for American Progress during the Trump administration, does not have a long history with Austin, a retired four-star general. Yet Magsamen immediately established “a trusting and loyal rapport” with the notoriously private Pentagon chief — no easy task, Kahl said.

“He could have seen her basically as someone who was close to the White House and therefore not, ‘his person,’” Kahl said. “She earned it by doing a great job, being loyal and being effective, both inside the building, in kind of helping him run the building, but also in the interagency: interacting with other chiefs staff and with the White House.”

Magsamen’s name only appeared in the press in January, when she waited to inform various officials in the Pentagon — including Austin’s No. 2 — and the White House about Austin’s hospitalization. The two-day delay prompted a storm of controversy from the White House to Capitol Hill. White House Chief of Staff Jeffrey Zients ordered a review by Cabinet secretaries on how they delegate authority. The Pentagon later implemented changes to its policies for informing senior officials and the public about the defense secretary’s condition.

Austin entered the hospital on Jan. 1. Magsamen and other top staffers learned of the hospitalization on Jan. 2, but didn’t inform the White House until Jan. 4.

The Defense Department said at the time that the holdup happened because Magsamen was ill and unable to inform the relevant officials quickly. Austin publicly took responsibility and apologized for the incident, and did not discipline any staffers.

Kahl gave Magsamen credit for playing an important role in reestablishing strong civilian leadership in the Pentagon, after many experienced civilians left during the Trump administration. During crucial meetings, including with Austin’s international counterparts, Magsamen often sat next to Austin, a sign of her senior position.

Magsamen is not universally liked around the Pentagon. Some officials who have worked with her say she can be demanding of the staff, and is known to be glued to her phone at all times. But some argue that’s just part of the job of a DOD chief of staff.

“Yes, she is demanding, she is the chief of staff for the secretary of defense of the most powerful military in the world. I don’t think you want somebody in this job who is not demanding,” said John Kirby, strategic coordinator for the National Security Council who worked with Magsamen in his previous role as DOD spokesperson. “You want somebody who works hard and pushes not just others but pushes herself.”

Others say Magsamen has been a crucial asset for Austin, and one of just a few advisers he trusts completely.

From managing the evacuation of Afghanistan, to Russia’s invasion of Ukraine, to the Oct. 7 Hamas attacks on Israel, Magsamen “came in on day one and has really been an indispensable adviser to Secretary Austin and a key part of frankly all of the mechanics of the DOD over the last 3.5 years,” said Mara Karlin, who served in several senior roles at DOD until December.

“She has worked harder in support of the department and realizing Secretary Austin’s agenda in the last 3.5 years than I could imagine any human being working,” she added.

“It was like that guy spinning plates on a stick, you know, and she had to spin a whole heck of a lot of plates and keep them all going at the same time,” Kirby said.

Kahl said Magsamen’s reputation in the Pentagon was borne out of the realities of the job, not her personality — and suggested perhaps she made enemies in a male-dominated building because of her gender.

“The more effective you are in that position, the more antibodies in some cases you’ll produce,” Kahl said. “If Kelly Magsamen weren’t a woman, would people be making the same complaints about her?”

“If you filter by the structure of the job and whatever gender double standards there are, Kelly is not more disliked than I would expect.”