Washington (CNN) After a 35-year government career that spanned the world under six presidents, and even included a brief stint as acting secretary of state, the State Department's top career diplomat has announced his retirement.

"I've decided that it is time to take a break," Undersecretary for Political Affairs Thomas Shannon said in an interview Wednesday in his seventh floor suite of offices with two reporters, including one from CNN. "Thirty-five years is a long time. I've fought the good fight."

Though not unexpected, Shannon's retirement is sure to come as a blow to the State Department rank-and-file, particularly among the foreign service who has viewed him as a steady presence during the tumultuous first year of the Trump administration.

"He has kind of seen it through every angle," said Secretary of State Rex Tillerson, who made a surprise appearance during the interview to praise Shannon. "Thirty-five years of experience is not something you replace overnight."

The State Department has faced its lowest morale in years over a talent flight by senior foreign service officers and a perceived insular approach from Tillerson that has been interpreted by longtime employees as evidence they are undervalued.

Shannon stressed the decision to leave was entirely his and was the culmination of months of reflection and discussions with Tillerson.

Though he said Tillerson asked him to stay on in the job, the death of his mother last November and turning 60 last week convinced Shannon it was the right time to take stock of his life and make a change.

"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, adding that Tillerson was "very gracious" about respecting his decision to retire.

To underscore the point, Tillerson called Shannon a "senior statesman" and role model for the foreign service, who was also respected throughout the West Wing of the White House.

"He is going to be really missed," he said of Shannon. "Having said that, I know this is something he has really been thinking about for some time."

Shannon joined the Foreign Service in 1984 as a political offer in Guatemala before going on to hold diplomatic posts across Africa and South and Central America.

He was the top diplomat for the Western Hemisphere at both the State Department and the National Security Council under President George W. Bush and served as the US ambassador to Brazil before former Secretary of State John Kerry hired him as counselor to the State Department, the first foreign service officer to hold the post in more than 30 years.

In February 2016, President Barack Obama nominated Shannon as undersecretary for political affairs, the third-top position at the State Department.

"When I took this job, I knew I was the designated survivor," he said of his nearly two years in the post. "Aside from helping the Obama administration to the finish line, I knew my job was also about helping this institution navigate the political transition (and) get across the river. And once across that river, help this institution -- the foreign service and civil service -- be responsive to our elected leadership."

After President Donald Trump's inauguration, Shannon served as acting secretary for two weeks before Tillerson was confirmed and then stepped in as his acting deputy for several months until Deputy Secretary John Sullivan arrived at Foggy Bottom. He promised Tillerson he would stay on for a year under his leadership.

"I thought giving it a year made sense and would allow me to have maximum impact," he said.

Tillerson tasked Shannon with several major initiatives since taking office, including leading talks with Russia to address so-called "irritants" in the relationship.

"The thing I will miss the most is, he is a walking encyclopedia," Tillerson said of Shannon, praising his "deep well of knowledge, experience and perspective" which he said he found invaluable during his first few months in office as he acclimated to the top job.

Tillerson recalled holding Saturday morning strategy sessions in which he said Shannon's counsel and institutional memory on how policies evolved over time helped him craft his own strategies. It's not yet clear whether Shannon's replacement will be a career State Department veteran, who have traditionally held the position, or a political appointment.

Shannon agreed to stay on the job until a replacement is identified and confirmed.

Tillerson said he told Shannon if he ever wanted to return "you just throw your hat back in the ring over here and it is going to land on somebody's head right away."

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

Shannon is one of an elite group of diplomats who rose to the top ranks of the foreign service as career ambassador. Out of five diplomats who held the rank at the beginning of the Trump administration, Shannon's departure will leave only one -- Stephen Mull.

But Shannon stressed it was important to make space for young, future leadership. He pointed to a generational change at the State Department, where 60% of the workforce has been working at the agency for 10 years or less.

"It helped me make this decision because I have supreme confidence in their capabilities and their willingness and abilities to assume leadership positions," he said.

Looking back on more than three decades of service, Shannon said his most transformative assignment was as a young labor officer in Johannesburg, South Africa, during a period that saw the election of Nelson Mandela.

"I participated in this incredible political transition, it was negotiated, it was peaceful, it was relatively non-violent, and it led to the election of a man for president who spent 27 years in prison. I got to play a role at a grassroots level and see democracy for what it can be," he recalled.

Shannon expressed hope that future generations would recognize the importance of service to America that seems lacking in today's "hyper-politicized environment."

"It's been an enormous honor of serving my country through four decades of some of the most momentous political change that one can imagine," he said. "I could not have been happier or more blessed."