The well-respected combat commander served over four decades in the military and has extensive experience leading troops in the Middle East. In more recent years, Mattis has served as a NATO commander, then moved on to head the US Central Command under President Obama, before retiring in 2013.
Mattis will run the Pentagon and work alongside Trump's new national security advisor Michael Flynn and CIA director Mike Pompeo less than four years after leaving active service. Congress will need to pass special legislation so that the retired general can serve in the position, as ex-military officers aren't supposed to take the job unless they've been out of the game for at least seven years.
Mattis, like Trump, has made headlines for some controversial outbursts, like the time he said that "it's fun to shoot some people" at a panel in San Diego in 2005. He's also been an outspoken critic of the Iran nuclear deal and has given Trump some insight about waterboarding.
Mattis reportedly told Trump, "Give me a pack of cigarettes and a couple of beers and I do better with that than I do with torture," according to the president-elect.
Watch: Donald Trump's Day One Agenda