The eldest of three boys, Shanahan was born in 1962 to Michael and Jo-Anne Shanahan. Michael Shanahan was a law enforcement officer and a Vietnam vet, who was awarded a Bronze Star Medal for his service.
"Growing up, my understanding of the war came from my dad, his friends and the few stories they would share," Shanahan said in an article posted to the Department of Defense's website.
Source: The Department of Defense
During his confirmation hearing before the Senate Armed Services Committee, Shanahan said that his father had also instilled a creed of "service before self" in him and his younger brothers as they grew up.
Source: Senate Armed Services Committee
As deputy defense secretary, Shanahan kept a picture of his father hanging on the wall of his office, just over his father's Bronze Star.
Source: The Department of Defense
A native of Washington state, Shanahan attended the University of Washington, where he earned a degree in mechanical engineering. He remained involved with the school later in life, too. Starting in 2012, he served on the university's board of regents for five years.
Shanahan's education didn't stop there, though. He went on to earn both a master's in mechanical engineering and an MBA from MIT as well.
Source: US Department of Defense
There, he led projects like Boeing Missile Defense Systems and Boeing Rotorcraft System.
Source: US Department of Defense
According to The New York Times, Shanahan was widely credited with saving the company's troubled 787 Dreamliner project in 2007.
Source: The New York Times
He rose to the rank of Boeing's senior vice president of supply chain and operations.
Source: US Department of Defense
His political donations appear to have been fairly even-handed between the two major parties. Between 1990 and 2016, Shanahan donated $6,250 to Republican and conservative causes and politicians, and $5,000 to Democratic and liberal causes and politicians.
Source: Center for Responsive Politics
According to the Center for Responsive Politics, he donated nothing to Trump, Trump's campaign committee, and other Trump allies.
Source: Center for Responsive Politics
Less than a year after squabbling with Boeing over the cost of Air Force One, Trump announced that he planned to appoint the longtime aerospace exec to the position of deputy director of defense on March 16, 2017.
Source: GeekWire, Business Insider
During the proceedings, the late Senate Armed Services Committee chairman John McCain accused Shanahan of dodging questions about supporting Ukraine in their conflict with Russia.
Shanahan said he and Mattis developed a good working relationship at the Pentagon. "He understands the lethal part and I am the engineer who can get it," he said in an article on the Department of Defense's website.
Source: The Department of Defense
He also called the talent in the Department of Defense "incredible." Shanahan will take over from Mattis starting on January 1, 2019.
Source: The Department of Defense
In an article posted on the Department of Defense's website, Shanahan said that the "operational" side of working at the Pentagon wasn't intimidating, but added, "The difference is the consequences. If you get something wrong here, it’s huge."
Source: The Department of Defense