Just before noon, Obama took the oath on the steps of US Capitol's West Front, dressed in red, white and blue, in a spectacular inaugural ceremony watched by an estimated 700,000 flag-waving people crowding the national mall and millions more on television at home.
Raising his right hand, Obama placed his left hand on two bibles, one which his idol Abraham Lincoln used for his first inauguration in 1861 and the other, a "travelling Bible" of legendary American civil rights leader Martin Luther King Jr, to take the oath a second time in two days from Chief Justice John Roberts.
Obama swore he would "faithfully execute the office of president of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States," as he had done in a quiet ceremony in the gilded Blue Room of the White House Sunday.
The ceremony Sunday in the presence of only First Lady Michelle and daughters Malia and Sasha was designed to be on the right side of law as under the US Constitution the term of a new president begins at noon Jan 20.
The pomp and show had to thus wait till Monday as Jan 20 fell this time on a Sunday.
Monday's ceremony was steeped in symbolism as the two bibles represented for the son of a Kenyan father and an American mother "two people whose shoulders he's standing on" for the inauguration.
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It also carried added symbolic resonance as the second swearing in of America's first black president came 150 years after Lincoln's emancipation proclamation and almost 50 years since King's famous "I have a dream" speech.
Before Obama, Vice President Joe Biden also took the oath of office administered by Supreme Court Justice Sonia Sotomayor as he had done Sunday at his official residence.
Speaking at a reception for supporters late Sunday, Obama suggested he would dwell on the "common good" and the "goodness, the resilience, neighbourliness, the patriotism" of Americans.
"What we are celebrating is not the election or the swearing-in of the president," Obama said. "What we are doing is celebrating each other and celebrating this incredible nation that we call home."
After Monday's ceremony, Obama will have the traditional lunch with US lawmakers in the Capitol building's Statuary Hall and then walk at least part of the parade route down Pennsylvania Avenue toward the White House.
(Arun Kumar can be contacted at arun.kumar@ians.in)