Mueller, who took over the FBI a week before the Sept. 11, 2001, terrorist attacks, had a relentless style of management that began with 7 a.m. briefings each day. He avoided the spotlight and was credited with transforming the FBI and its mission.
Mueller served 12 years in the position, working for President George W. Bush and then
After the Sept. 11 attacks, Mueller worked to transform the bureau into an intelligence-driven agency designed to prevent rather than react to terrorism.
"Bob will be known as the most transformative director in the history of the FBI since Hoover," former Homeland Security Secretary Michael Chertoff told USA Today in 2013. "And I mean that in a good way."
Throughout his career, Mueller, now 72, has moved in and out of private law practice. He once gave up a lucrative salary at a private law firm to become a homicide prosecutor for the federal government.
He and then-Deputy Atty. Gen.
Some members of Congress criticized Mueller when he headed the FBI for failing to share more information with the public.
The bombing of the 2013
Mueller was born into a wealthy Manhattan family, graduated from Princeton University and received a law degree from the University of Virginia.
During the Vietnam War, he served as a Marine, led a rifle platoon and won a Bronze Star. He became chief of the FBI's criminal division under President George H.W. Bush and served as a federal prosecutor in San Francisco and Boston.
Northern California criminal defense lawyer Cristina Arguedas, who represented defendants in federal court in San Francisco, told the Los Angeles Times in 2001 that Mueller and the FBI were "the perfect marriage of a man to a job."
"He's a law enforcement advocate to his bones," Arguedas said. "He lives it and breathes it."
He was well-known for walking the halls of his San Francisco office early in the morning and late into the evening, doing mock roll calls to check on his staff.
Some viewed Mueller as overbearing, but former federal prosecutor Joseph Russoniello said his style was a product of his military training and self-control.
Michael Burt, then a San Francisco deputy public defender who worked on a death penalty case Mueller prosecuted, described Mueller to the San Francisco Chronicle in 2001 as "one of these Jimmy Stewart characters, with old-fashioned American values."
Sen. Dianne Feinstein of California praised Mueller when President George W. Bush was considering appointing him as head of the FBI. Mueller is a Republican, and Feinstein is a Democrat.
"I know Bob, both personally and professionally, and I believe him to be an individual of the highest integrity," she wrote in a letter to the president.
Mueller most recently was deeply involved in the Volkswagen litigation. A federal judge in San Francisco overseeing more than 500 lawsuits against the German automaker over excess diesel emissions named Mueller "settlement master."
Mueller oversaw settlement talks involving the car company, the government and plaintiffs' lawyers.
He also did the internal investigation for the NFL on how the league botched its handling of the Ray Rice domestic violence case.
His report said the NFL commissioner,
Mueller's report also faulted the NFL for failing to thoroughly investigate the incident.
5:50 p.m.: This article was updated with additional background on Mueller.