U.S. Secretary of Defence Jim Mattis addresses a news conference during a NATO defence ministers meeting at the Alliance headquarters in Brussels, Belgium, February 15, 2018. REUTERS/Francois Lenoir - RC11E611BC10

Only one senator voted against confirming retired Gen. James Mattis as defense secretary.  (Reuters/Francois Lenoir)

Who is James Mattis? 5 things to know about the defense secretary

For more than a year, James “Jim” Mattis has been President Trump’s secretary of defense.

In his position, Mattis, 67, has bucked with the administration in advocating for the U.S. to stay in the Iran nuclear deal and backed Trump’s plan to restrict transgender individuals from serving in the military, according to the White House.

Mattis has a storied history serving in the U.S. Marines. Additionally, he served on the board of Theranos, an embattled health technology company. He assumed office as the nation’s defense secretary on January 20, 2017.

Here are five things to know about the defense secretary.

Mattis has served in the military since he was 18

U.S. Defense Secretary Jim Mattis lands in Kabul on March 13, 2018 on an unannounced trip to Afghanistan. REUTERS/Phil Stewart - RC1855585A80

Jim Mattis joined the Marine Corps Reserve when he was 18 years old.  (Reuters/Phil Stewart)

Mattis served in the U.S. Marines for more than 40 years, beginning at the age of 18 when he joined the Marine Corps Reserve, according to his Defense Department biography.

Born in Richland, Washington, about 200 miles southeast of Seattle, Mattis graduated from Central Washington University in 1971 and became a second lieutenant in the Marine Corps after school, his biography said.

He’s served in Afghanistan and Iraq and “led all U.S. Marine Forces in the Middle East as Commander, I Marine Expeditionary Force and U.S. Marine Forces Central Command,” according to his biography. He was also in charge of the 1st Marine Division in the “initial attack and subsequent stability operations in Iraq in 2003.”

Additionally, Mattis served as NATO’s Supreme Allied Commander from 2007 to 2009.

Obama appointed him to commander of U.S. Central Command

Mattis took over as the commander of the U.S. Central Command in 2010, according to an archived release from the Marines.

“His insights into the nature of warfare in the 21st century significantly influenced my views about the shape and posture of our armed forces for a complex and unpredictable future,” then-defense secretary Robert Gates said of Mattis at the time.

He was officially appointed to the position by former President Barack Obama to replace Gen. David Petraeus.

In the position, Mattis oversaw more than 200,000 service members, according to his Defense Department biography.

He retired from the Marine Corps in 2013.

Mattis needed a special waiver to lead DOD

U.S. Defense Secretary James Mattis joins White House Press Secretary Sarah Huckabee Sanders for the daily press briefing at the White House in Washington, U.S. February 7, 2018.  REUTERS/Jonathan Ernst - RC14B2474D80

Defense Secretary Jim Mattis was almost unanimously approved for his position by the Senate.  (Reuters/Jonathan Ernst)

Before he was confirmed by the Senate to lead the Defense Department, the retired general needed a special waiver.

According to the National Security Act of 1947, the secretary of defense needs to “be appointed from civilian life.” The law said the nominee must be out of active duty service for at least 10 years. However, Congress changed the law from 10 to seven years in 2008, NBC News reported.

The House and the Senate ultimately passed the waiver with several Democrats opposing. 

Before the waiver was granted, Trump said, “There are going to be a lot of angry people” if it wasn’t passed.

Only one senator voted against his confirmation

Sen. Kirsten Gillibrand, D-N.Y., was the only senator to vote against Mattis’ confirmation. Jeff Sessions, then a Republican senator from Alabama, was the only member not to vote.

Gillibrand met with Mattis privately, but ultimately decided to vote against a waiver that would allow him to lead the Defense Department.

“He has served our country admirably,” Gillibrand said, according to The Hill. “He is well-regarded as an extraordinary general, and I am very grateful for that service, and I’m very grateful that he’s willing to continue his service for the president-elect. But I still believe that civilian control of our military is fundamental to the American democracy.”

Mattis apparently isn’t a fan of his iconic nickname

Known for his sometime colorful rhetoric – called “Mattisisms” – the retired general has earned the nickname “Mad Dog Mattis.”

But during his Senate confirmation hearing, Mattis revealed that he might not be the biggest fan of the moniker.

“Senator, I assure you that nickname was given to me by the press and some of you have experienced similar occasions with the press where they perhaps they didn't get it quite right,” Mattis said.

The Associated Press contributed to this report.

Kaitlyn Schallhorn is a Reporter for Fox News. Follow her on Twitter @K_Schallhorn.