Nobel Peace Prize victor Sirleaf, 79, is stepping down after two terms in office that brought the impoverished country out of back-to-back civil wars and saw it grapple with a deadly Ebola outbreak.
He beat 73-year-old Joseph Boakai in the presidency race.
However, the Electoral Commission of Liberia are yet to declare the official results of the presidential votes.
Former world footballer of the year George Weah has won Liberia's presidential election.
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He lost to Sirleaf, the president he will replace, in the second round of voting in 2005 and unsuccessfully ran for vice-president in 2011.
Boakai waged a bitter legal battle over problems with queue control and voter identification in the October 10 first round.
Weah retired from professional football in 2003 and soon became involved in politics when he formed the Congress of Democratic Change in 2005. The failure of any candidate to secure an outright majority forced the run-off.
The announcement made by the National Elections Commission chairman Jerome Karyoka comes after premature calls on social media by some sections of the media and prominent personalities that ex-football star George Weah had won the election.
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Charles Taylor is serving a 50-year sentence in Britain for war crimes committed in neighbouring Sierra Leone, but his presence has loomed over the election.
He entered politics after his retirement and is now a senator in Liberia's parliament.
The vote is the first time in more than 70 years that the West African nation founded by freed American slaves is seeing one democratically elected government hand power to another.
He and Boakai voted in polling stations near their homes in Paynesville, a suburb of Monrovia, the capital of this nation of 4.6 million people.
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