|Ellen Johnson Sirleaf is Africa's first elected female head of stat|
With 86% of the votes counted, Mrs Sirleaf is on 91% after her rival, Winston Tubman, boycotted the election.
He alleged that the poll - seen as a test for Liberia's fragile democracy - was rigged in Mrs Sirleaf's favour. Foreign observers condemned his decision to boycott the election, saying it was well-organised. Turnout in Tuesday's poll was 37.4% - about half the number who turned out for the election's first round last month.
Mrs Sirleaf won the most votes in that round, but failed to gain an outright majority, forcing a run-off.
With such a low turnout and no majority in parliament, her task promises to be all the more difficult, says the BBC's Thomas Fessy in the capital, Monrovia.
Mrs Sirleaf was awarded the Nobel Peace prize shortly before the first round for helping to end Liberia's 14-year-civil war.
Mrs Sirleaf said she would reach out to all presidential candidates to work within an inclusive government.
"What I will offer them is not yet known," she told reporters.
"I am confident we will be able to reconcile the people."
The election commission said Mr Tubman, whose name remained on ballot papers, got 9% of the vote.
He told the BBC he rejected the result, but his Congress for Democratic Change (CDC) would consider any offer from Mrs Sirleaf and her Unity Party (UP) to serve in the government.
"I will stick with my party and maybe we can find someone in our party who can deal with the Unity Party government and Mrs Sirleaf to bring about reconciliation," he told the BBC's Network Africa programme.
"But I have seen nothing in Mrs Sirleaf's past and even her present that [shows] she wants to reconcile the country."
Former warlord Prince Johnson, who came third in the first-round with 12%, urged his supporters to back Mrs Sirleaf in the second round.
The run-up to Tuesday's voting was marred by violence in which two people were killed when security forces opened fire. Mrs Sirleaf has promised an investigation.
International observers, including those from the Economic Community of West African States (Ecowas), said the election process was credible.
"The mission wishes to state that it found no major irregularities in the voting process itself. It considers, on the whole, that the elections of 8 November met the acceptable conditions of being free, fair and transparent," the Ecowas observer team said in a statement.
Mrs Sirleaf first won elections in 2005, following the end of a civil war that saw former leader Charles Taylor - now on trial at The Hague for war crimes - forced out of power.