Obama indicated he was, and Roberts started reciting -- and Obama repeating -- the 35-word oath that is prescribed by the Constitution.
But at one point early on, Obama paused, as if grasping for the next words. Roberts helped him over the brief awkward moment, repeating a few words to get Obama back on track.
He was then the first to congratulate Obama on his new job.
The inaugural oath is the chief justice's sole responsibility on Jan. 20 -- although it is a traditional role, not one set forth in the Constitution. But the affable Roberts and his conservative-leaning Supreme Court could have much to say in the years to come about Obama's most important policy choices.
Former President George W. Bush left the court with two relatively young and reliably conservative voices, those of Roberts, 53, and Justice Samuel Alito, 58. Roberts took his seat in 2005 and Alito joined him the next year.
Roberts is the youngest chief justice in more than 200 years. He easily could still be in his role a quarter century from now, long after Obama has left office.
He and Obama are similar in many ways. Both are late baby boomers. Roberts is 53, Obama 47. And both got their law degrees from Harvard and made rapid ascents to power. But their politics diverge sharply.
Roberts was an official in Republican administrations before becoming an appeals court judge and then chief justice under Bush.
Obama was one of 22 Senate Democrats to vote against Roberts' confirmation to the Supreme Court in 2005 -- the first time a Supreme Court justice has sworn in a president who voted against him.
As president, Obama will try to use Supreme Court vacancies to counter Roberts' influence, either by replacing aging liberals with justices as young as or younger than Roberts or by changing the court's balance if a conservative justice retires unexpectedly.
Obama added the words "so help me God" to the end of the constitutional oath, following a practice established by George Washington and followed by most presidents.
The last time a chief justice swore in a president of a different party was in 1997, when Chief Justice William H. Rehnquist, a Republican, swore in Democrat Bill Clinton for a second term. Two years later, Rehnquist would preside over Clinton's impeachment trial in the Senate which resulted in an acquittal.
The Lincoln Bible used by Obama was on loan from the Library of Congress.