White House Chief of Staff John Kelly will be leaving his position at the end of the year, President Trump announced on Dec. 8. Days later, Trump confirmed Office of Management and Budget Director Mick Mulvaney would replace him as acting chief of staff.

In a Dec. 14 tweet, Trump hailed Kelly as a "great patriot" and thanked him for his service.

"I am pleased to announce that Mick Mulvaney, Director of the Office of Management & Budget, will be named Acting White House Chief of Staff, replacing General John Kelly, who has served our Country with distinction," he tweeted, in part.

Kelly, a retired Marine general, was one of Trump’s first Cabinet picks – but he hasn’t had the same role throughout the administration.

MICK MULVANEY TO REPLACE JOHN KELLY AS TRUMP'S ACTING WHITE HOUSE CHIEF OF STAFF: 5 THINGS TO KNOW

First tapped to lead the Department of Homeland Security, Kelly, 68, became Trump’s White House chief of staff in July 2017.

“He’s a Great American and a Great Leader,” Trump said on social media at the time. “John has also done a spectacular job at Homeland Security. He has been a true star of my Administration.”

The decorated military veteran was brought on to bring order to what was seen as a chaotic White House. Since becoming chief of staff, Kelly has worked to create a formal line of authority and decision-making within the administration and reform the security clearance process – something that has found him at odds with Jared Kushner, the president’s son-in-law.

Read on for a look at five things you should know about Kelly.

Trump originally appointed him as secretary of the Department of Homeland Security

While still president-elect, Trump originally picked Kelly to lead the Department of Homeland Security. Trump’s transition team said Kelly would “spearhead the urgent mission of stopping illegal immigration and securing our borders,” according to The Guardian.

While overseeing the Department of Homeland Security, Kelly took lead on some of Trump’s more controversial actions, including the travel ban of people from Muslim-majority countries.

He assumed the Cabinet office in January 2017, but was abruptly named Trump’s chief of staff six months later in July, replacing Reince Priebus.

He is a Gold Star father

Both of Kelly’s sons, Robert and John, joined the Marines. Robert died in Afghanistan in 2010 at the age of 29.

Kelly was widely praised by those who organized a memorial event for Massachusetts military members who died since the 9-11 attacks. Kelly accepted an invitation to give a speech at the 2010 event, held just three weeks after his son died. But he didn’t mention that he, too, was a Gold Star father, according to the Boston Globe.

“Could you find a classier person? He’s a great man.” — Marine veteran Chris Lessard

“That was my first impression of General John Kelly,” Marine veteran Chris Lessard told the newspaper. “Could you find a classier person? He’s a great man.”

His status as a Gold Star father was brought into the spotlight as Trump’s chief of staff. Kelly took the helm of a White House press briefing in October 2017 to defend the president’s phone call to the widow of a Green Beret soldier killed in Niger. Trump’s phone call to the widow was criticized by Rep. Frederica Wilson, D-Fla., who listened into the conversation and later attacked the president's sentiments.

As a teenager, Kelly hitchhiked across the country

Kelly grew up as in an Irish-Catholic family in Boston.

As a teenager, he hitchhiked across the country multiple times before he was 16 years old, according to the Marines website. He also rode in an empty boxcar on a freight train from Seattle to Chicago before he turned 16, according to the Marines.

He oversaw the transfer of Guantanamo prisoners for Bowe Bergdahl

As commander for the U.S. Southern Command, part of Kelly’s duties included overseeing operations at Guantanamo Bay military prison.

During his time there, Kelly also oversaw the transfer of prisoners for Bowe Bergdahl, the Army sergeant who went missing from his remote infantry station near the Pakistani border in 2009 and was captured by the Taliban.

The swapping of five Taliban prisoners for Bergdahl was controversial, but Kelly said in a 2016 Defense Department press briefing that he “follow[s] orders.”

“That's a policy decision to transfer them,” Kelly said, according to a transcript of the briefing. “I know it's caused a lot angst in a lot of areas. I don't -- I don't try to slowdown transfers. I -- I facilitate transfers.”

He also said the transfer was “dicey” because of the amount of press there at the time. Kelly often criticized media coverage of the detention center, the Miami Herald reported in 2015.

Kelly retired from the U.S. Southern Command in January 2016, having served in the military for more than four decades.

Kelly has an extensive career history with the Marines

Kelly joined the Marines in 1970 after his mother told him his draft number would be coming up, the Boston Globe reported.

He attended school at the University of Massachusetts after he was discharged as a sergeant in 1972, according to his Defense Department biography. But after his college graduation, he joined the 2nd Marine Division, according to the biography.

He served on sea duty in Florida, joined the U.S. Army’s Infantry Officer Advanced Course in Georgia and was stationed in Washington, D.C. He studied at Quantico and the National War College.

It was while Kelly was the Commandant’s Liaison Officer to Congress in the 1990s that he became a colonel, his biography said.

Kelly was stationed in Belgium in 1999, serving as the special assistant to the Supreme Allied Commander Europe before serving multiple tours in Iraq.

Although retired, Kelly was prepared to serve for either Trump or Democratic presidential nominee Hillary Clinton if asked, Time reported. When he got the call from the incoming Trump team – while he was apparently watching college football – Kelly reportedly asked his wife what he should do.

According to Time, his wife said, “If they think they need you, you can’t get out of it. Besides, I’m really tired of this quality retired time we’re spending together.”

The Associated Press contributed to this report.