Harvard-trained economist Johnson-Sirleaf beat football millionaire George Weah in last year's elections, and must employ international goodwill and aid to rebuild a shattered nation lacking even basic services like water and power.
"The United States has a long and special relationship with Liberia which we will continue," a US official with Laura Bush told reporters on the way to Monrovia ahead of the inauguration.
The official said it was significant Johnson-Sirleaf was being installed as the United States marked Martin Luther King Day, because freed American slaves had helped to found Liberia in 1847.
Johnson-Sirleaf's election was "a clear example of women's empowerment across Africa", the official said, but acknowledged the task she faced after a 14-year civil war, saying "the president-elect will have to show results rather quickly".
"America must help build our country, because Liberia is an offspring of America ... If they fail to help this new government and this country, God will hold them accountable"
Victory Sieah, a 46-year old mother
Liberians, weary of their country's recent history of conflict, chaos and destruction, have high expectations for the new president they call "Mama Ellen" and for the superpower across the Atlantic they look upon as a kind of Godfather.
Links between the United States and Africa's oldest independent republic still run deep.
"America must help build our country, because Liberia is an offspring of America ... If they fail to help this new government and this country, God will hold them accountable," said Victory Sieah, a 46-year-old mother of two children.
US officials say Washington spent more than $840 million last year on Liberia as it emerged from a brutal civil war that ended in 2003 after killing 250,000 people and leaving the country's infrastructure in ruins. They say helping the country is a priority of President George Bush's administration.
US secret service agents mingled with UN peacekeepers in the ramshackle capital and two American warships, the USS Mount Whitney and USS Carr, were in position offshore.
Across the city, rubbish has been cleared, bullet-scarred walls painted over and trees planted in a face-lift ahead of the ceremony, at which a group of African leaders were expected, too.
Liberia is rebuilding from a brutal
civil war that ended in 2003
Analysts say Johnson-Sirleaf's vows to fight bribe and keep the peace may fail unless donors make good on pledges of aid.
"They must now put money on the table," the International Crisis Group said in a report on Friday.
"If Johnson-Sirleaf is not helped to solidify her administration in the next months, there will be opportunities for detractors, ex-combatants and other spoilers to try their luck with new violence," the Brussels-based think tank said.
Johnson-Sirleaf is also under growing pressure to act on Charles Taylor, the Liberian ex-President, in exile in Nigeria since 2003 and indicted on 17 counts of war crimes in Sierra Leone.
Rice said Johnson-Sirleaf, a 67-year-old former finance minister, understood that Taylor had to be brought to justice.
"Charles Taylor is out of Liberia, he is through raping and pillaging his country and the Liberian people are trying to look forward. I really think we owe it to them to look forward and not back," Rice told reporters on her way for Monrovia.