A 73-year-old retired Marine Corps officer and former prosecutor, the silver-haired Mueller is one of the few insiders to have emerged unscathed from the ravages of partisan enmity in Washington.
Named special counsel in May to investigate charges that members of Donald Trump's presidential campaign may have colluded with Russia, Mueller's months-long probe began showing its first results on Monday.
Trump's former campaign manager Paul Manafort, 68, and business partner Rick Gates, 45, were charged with allegedly hiding millions of dollars they earned working for former Ukrainian politician Viktor Yanukovych and his pro-Moscow political party.
George Papadopoulos, a Trump campaign aide, pleaded guilty meanwhile to lying to FBI investigators probing the campaign's possible links to Russian interference in the 2016 election.
As special counsel, Mueller is empowered to pursue not only Russian interference but any other crimes his large team of prosecutors should uncover.
Trump has consistently rejected any suggestion of ties between his camp and Moscow as "fake news" and tweeted again on Monday there was "NO COLLUSION."
Born August 7, 1944, in New York City, Robert Swan Mueller III, was nominated to head the Federal Bureau of Investigation by then-president George W. Bush in 2001.
He was unanimously confirmed by the Senate and took the job just a week before the September 11 Al-Qaeda attacks on New York City and Washington.
Nicknamed "Bobby Three Sticks" for the suffix after his name, Mueller went on to serve as the nation's top cop for 12 years.
President Barack Obama extended his 10-year term for an additional two years and Mueller finally stepped down as FBI chief in 2013.
Bronze Star in Vietnam
Mueller was replaced as FBI chief by James Comey, whose abrupt firing by Trump in May was among the acts which led to Mueller's appointment as special counsel.
Mueller grew up on Manhattan's tony upper East Side and earned a bachelor's degree from Princeton University in 1966.
After graduating from the elite Ivy League school, Mueller joined the Marines, and after a year as an enlisted man, entered officer candidate school.
As a Marine platoon commander serving in Vietnam, Mueller earned a Bronze Star for valor and a Purple Heart for wounds received in combat.
Following his military service, Mueller earned a law degree from the University of Virginia and went on to serve as a US Attorney in Massachusetts and California.
Two of his most heralded prosecutions involved New York mobster John Gotti and General Manuel Noriega of Panama.
Mueller has a reputation as a no-nonsense straight arrow who works long hours, shuns the limelight and has little tolerance for slack job performance.
"He was the boss, and people understood it," former FBI deputy director Oliver Revell said.
Among those praising his qualities at the time of his 2001 nomination as FBI chief was then-Republican senator Jeff Sessions, who is now Trump's attorney general.
"His skills should be a perfect match for the challenge," Sessions said.
Mueller is married to the former Ann Cabell Standish, a school teacher.
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