Mr Ashe is accused of taking $1.3m, spending the cash on luxury goods.
UN Secretary General Ban Ki-moon said he was "shocked and deeply troubled" by the allegations.
"In return for Rolex watches, a basketball court and bespoke suits, John Ashe sold himself and the global institution he led," said US Federal Attorney Preet Bharara. "United by greed, they converted the UN into a platform for profit."
Prosecutors say Mr Ashe used his position as permanent resident to the United Nations for Antigua and Barbuda and General Assembly head to introduce a UN document supporting a multibillion-dollar UN-sponsored conference centre that Mr Ng hoped to build as his legacy in Macau, where he lived.
The conference centre was supposed to function as a satellite operation for the world body.
The scheme unfolded between 2011 and 2014, including Ashe`s tenure as General Assembly head, prosecutors said.
Mr Ashe also set up meetings with government officials in Antigua and Kenya to help the real estate developers land big development contracts, Mr Bharara said.
He is also said to have evaded tax on the bribe money he received and allowed the businesspeople pay for him and his family to stay at an $850-a-night hotel in New Orleans.
Mr Ashe was arrested on Tuesday. Five others including Mr Ng are also being held.
They include another diplomat, Francis Lorenzo from the Dominican Republic, and two naturalised US citizens who live in China who allegedly helped facilitate the scheme, who have been charged with offences including bribery of a UN official and conspiracy to launder money.
Mr Bharara said the investigation was continuing and more arrests were likely.
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