"I thank the Liberian people for performing their legal duty and I am happy to be the next president of Liberia," said Mrs Sirleaf.
Dressed in a burgundy robe and headscarf, she told reporters after a ceremony certifying the November 8 results at the Centennial Pavillion in downtown Monrovia that it was an historic moment for Liberia.
The Harvard-educated banker, 67, earned 59.4 percent of votes cast to best football hero George Weah in a run-off aimed to help Liberia turn the page on decades of lawlessness and corruption after back-to-back civil wars since 1989.
She will be inaugurated January 16, as will the new bicameral legislature for Africa's oldest independent republic, settled in 1847 by freed American slaves.
Mr Weah has alleged massive vote fraud in the November 8 polls, and has launched a complaint process with the NEC that his party, the Congress for Democratic Change, has vowed to take to the Supreme Court.
The elections were declared peaceful and credible by a host of international observers.
Pressure has been mounting on Mr Weah, even from among his supporters, including two former warlords who endorsed his candidacy, to concede in the interests of peace.
Niger President Mamadou Tandja, current head of the 15-member Economic Community of West African States (ECOWAS), added his voice to the African chorus praising the elections and urging the dispute be resolved quickly and peacefully.
French President Jacques Chirac was among the first of Mrs Sirleaf's new international counterparts to extend his congratulations, releasing a public message that hailed the "new page in Liberia's political history" written with her election.
She has also been congratulated by UN chief Kofi Annan.
Mrs Sirleaf now faces the task of rebuilding a country shorn of infrastructure, with sky-high unemployment and threatened by the continued cross-border recruitment of young fighters to feed west Africa's enduring conflicts.
Mrs Sirleaf has vowed to serve just one term in office, couching her government as a transitional one to be backed up by the 15,000-strong UN peacekeeping mission on the ground since October 2003.