Everything you need to know about Trump's new chief of staff

President Donald Trump announced Friday via Twitter that he has selected retired Gen. John Kelly, secretary of the Department of Homeland Security, to be his new chief of staff.

Kelly will replace Reince Priebus, who served as chief of staff for six months and was formerly the longest-serving Republican National Committee chairman.

"I have been fortunate to have served my country for more than 45 years - first as a Marine and then as the Secretary of the Department of Homeland Security. I am honored to be asked to serve as the Chief of Staff to the President of the United States," Kelly said in a statement responding to the new position.

He also thanked those he worked with at the Department of Homeland Security. "To the tremendous men and women of DHS, I thank you for the opportunity to serve as your Secretary. When I left the Marines, I never thought I would find as committed, as professional, as patriotic a group of individuals. I was wrong. You accomplish great things every day defending our nation and I know your exceptional work will continue," Kelly said.

Kelly, a retired Marine four-star general, was confirmed as the secretary of homeland security on Jan. 20. As secretary, he was a proponent of the president's travel ban and implemented new regulations for laptops on planes in passenger carry-ons -- a ban that was recently lifted.

He first met with Trump, then president-elect, on Nov. 20 at a Trump-owned golf course in New Jersey and was the second general selected for the Cabinet after retired Gen. James Mattis was chosen for defense secretary.

As secretary for the Department of Homeland Security, established in 2002 in the wake of the 9/11 attacks, Kelly dealt with issues like immigration, border security, domestic terrorism threats and cybersecurity.

Kelly traveled twice to Mexico as secretary of homeland security, most recently in July, where he met with President Enrique Peña Nieto and other senior-level officials. He also traveled to Jordan and Saudi Arabia to discuss security issues with his majesty King Abdullah II of Jordan and Saudi Crown Prince and Minister of the Interior Mohammed Bin Nayef Abd Al-Aziz Al Saud.

He met with other leaders from Canada, Costa Rica, Honduras and Haiti during his time as secretary.

Here's what you need to know about Kelly:

Name: John Francis Kelly

Age: 67

Birthplace: Boston

What he used to do: He was the commander of U.S. Southern Command and a four-star general before he was confirmed as the secretary of homeland security. As head of SOUTHCOM, he was responsible for Guantanamo Bay and all U.S. military operations in South and Central America. At his retirement earlier this year, he was the armed forces' longest-serving general.

Military career:

Kelly enlisted in the Marine Corps in 1970. His first military deployment was to Guantanamo Bay in 1971 when he was just 20 years old.

He graduated from the University of Massachusetts before returning to the Marine Corps and working his way up the ranks, with stints on aircraft carriers, in the nation's capital and at Camp Pendleton in California.

In 1999 he served as the special assistant to the supreme allied commander in Europe, one of NATO's two strategic commanders. By 2002, he was promoted to brigadier general and served - mostly in Iraq - with the 1st Marine Division as the assistant division commander.

After three years as a legislative assistant to the Marine Corps commandant in Washington, D.C., Kelly was promoted to major general and returned to Camp Pendleton to command I Marine Expeditionary Force. During his deployment in 2008 to Iraq's Anbar and western Nineveh provinces, he was a key military player in what became known as the Anbar Awakening, which temporarily reduced sectarian violence.

Kelly commanded Marine Forces Reserves and Marine Forces North before becoming the head of SOUTHCOM in 2011.

Things you might not know about him:

Kelly is a Gold Star father. His younger son, Marine 1st Lt. Robert Kelly, was killed by an improvised explosive device in Afghanistan in 2010.

He has spoken movingly about the loss of his son and about how little some Americans know about the sacrifice of service members.

"Those with less of a sense of service to the nation never understand it when men and women of character step forward to look danger and adversity straight in the eye, refusing to blink or give ground, even to their own deaths," Kelly said in a Veterans Day speech in 2010.

"The protected can't begin to understand the price paid so they and their families can sleep safe and free at night. No, they are not victims but are warriors, your warriors, and warriors are never victims, regardless of how and where they fall. Death or fear of death has no power over them. Their paths are paved by sacrifice, sacrifices they gladly make - for you."

Where he stands on the issues:

Guantanamo Bay, Cuba

While Kelly was prepared to shut down the U.S. detention facility at the direction of President Barack Obama, he did not conceal his strong disagreement with the decision.

"They're detainees, not prisoners," Kelly told Military Times in January 2016. "The lifestyle they live in Guantanamo is - they can't simply be put in prison in the United States."

"Everyone has real, no-kidding intelligence on them that brought them there," he added. "They were doing something negative, something bad, something violent, and they were taken from the battlefield. There are a lot of people that will dispute that, but I have dossiers on all of them, built and maintained by the intelligence community, both military and civilian. There are no innocent men down there."

South and Central America

Kelly played a key role in negotiating the Alliance for Prosperity in late 2014.

The deal had the U.S. invest $1 billion in Honduras, Guatemala and El Salvador to "stimulate economic growth, reduce inequality, promote educational opportunities, target criminal networks responsible for human trafficking and help create governance and institutions that are transparent and accountable," according to the White House.

At Southern Command, he was vocal about the threat of "transnational organized crime" and warned that radicalized Muslims had left the Caribbean and South America to fight with ISIS in the Middle East.

Border security

Kelly is a proponent of stronger U.S. border enforcement but also believes that more can be done to reduce the number of people hoping to enter the U.S. without documentation, according to a published report.

"I think you have to have - we have a right to protect our borders, whether they're seaward, coastlines or land borders," Kelly told Military Times in January 2016.

"We have a right to do that. Every country has a right to do that. Obviously, some form of control, whether it's a wall or a fence. But if the countries where these migrants come from have reasonable levels of violence and reasonable levels of economic opportunity, then the people won't leave to come here," he said.

ABC News' Cheyenne Haslett and Dylan Wells contributed to this report.