The BBC's West Africa correspondent, John James, says Liberians regard the US as an old friend.
Everything in Liberia, which was founded by free slaves from America more than 150 years ago, draws its inspiration from the US "motherland" - including the flag, the government and local architecture, he says.
But Liberian infrastructure remains in a desperate state, with electricity and water being rare commodities.
Cheque book out?
"It is impressive the way that Liberia has decreased its debt, which was run up of course during years of conflict, and has had a high rate of GDP growth over the last years," Mrs Clinton said during a press conference with Mrs Johnson-Sirleaf.
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