John Roberts has been sworn in as the head of the US Supreme Court in a ceremony at the White House.
President George Bush's personal choice for the post of chief justice, he was earlier confirmed by a comfortable 78-22 margin in a vote in the Senate.
A Catholic with conservative views, Mr Roberts, 50, received the backing of many Democrats as well as Republicans.
"The Senate has confirmed a man with an astute mind and kind heart," Mr Bush said, standing next to Mr Roberts.
"All Americans can be confident that the 17th chief justice of the United States will be prudent in exercising judicial power, firm in defending judicial independence and above all a faithful guardian of the constitution," he added.
Mr Roberts spoke briefly, thanking the president and senators, and reiterating his oath, to "support and defend the constitution of the United States".
He was chosen to replace long-serving Chief Justice William Rehnquist, who died earlier this month.
With his appointment confirmed, attention will now shift to the replacement of Justice Sandra Day O'Connor, who is retiring.
Mr Bush is set to nominate her replacement imminently.
Born in Buffalo, raised in Indiana
Graduated from Harvard in 1979
Was clerk to Chief Justice Rehnquist and then served in the Reagan administration
Served on the District of Columbia appeals court
Argued before the Supreme Court 39 times
Democrats are expected to put up a stronger battle over that nomination, as Ms Day O'Connor is seen as holding a key swing vote.
Mr Roberts, a former appeals court lawyer and then judge, becomes the youngest chief justice for two centuries.
He watched the Senate vote on television at the White House, before having lunch with his wife Jane, President Bush and First Lady Laura Bush.
All 55 Republican senators voted for him, as did about half the Democrats in the Senate.
Republican Senate Majority Leader Bill Frist said: "Judge Roberts possesses the qualities that Americans expect in the chief justice of its highest court, and the qualifications that America deserves."
Dissenting Democratic leader Harry Reid said he had "too many unanswered questions about the nominee to justify confirming him to this lifetime seat".
Mr Roberts was sworn in immediately, ready to take his seat in time for a new Supreme Court session on Monday.
The Harvard graduate's youth could put him at the helm of the Supreme Court for a significant period of time, setting its tone for a generation.
Considered a low-key, modest man, John Roberts will hold one of the most influential posts in US public life.
The justices of the Supreme Court have immense power and are appointed until they die, resign or are impeached.
They can overrule politicians and declare the decision of the president unconstitutional.
In the near future the court is expected to consider some of America's most bitterly contested social issues, including assisted suicide, abortion, same-sex marriage, human cloning and campaign finance law.
Some campaigners have expressed concern at Mr Roberts' conservative views, though he has promised to enforce the letter of the constitution, not to be swayed by personal prejudices.
"My faith and my religious beliefs do not play a role," he told his Senate confirmation hearing.