The International Monetary Fund is ready to start cancelling Liberia's debt to the agency after securing sufficient funding for the process.
President Johnson-Sirleaf went to Washington to lobby for debt relief
The IMF says donor nations have pledged $842m (£407m) which will start the process once the pledges are honoured.
IMF chief Dominique Strauss-Kahn said it was a "critical step... toward comprehensive debt relief" for Liberia.
Liberia is struggling with the legacy of 14 years of brutal civil war, which ended in 2003, with 270,000 dead.
Liberia's deputy Information Minister Gabriel Williams told the BBC's Network Africa programme it was a "historic day" for Liberia.
"It enables our country to now move forward and accelerate the pace of reconstruction with the resources that we are going to be able to access as a result of this major development."
Liberia has found it difficult to get financial support because of a total international debt estimated to be $4.5bn.
The IMF debt relief plan involves a three-year growth programme to reduce poverty and help for Liberia to finance its remaining obligations.
The money to pay off the debts is understood to come mainly from existing IMF reserves, and an additional $71m from the G8 group of industrialised nations.
Last month President Ellen Johnson-Sirleaf visited Washington and argued Liberia had met political and economic requirements to qualify for help with its debts.
"President Johnson-Sirleaf has done a tremendous job in a difficult situation and this breakthrough will help all of us to offer her and Liberia more support," World Bank President Robert Zoellick said in a statement.