Thomas DiNapoli (left) said he was impressed by NI's skills base
A total of US $30m is to be made available for potential investment in Northern Ireland by the New York state retirement fund.
The state comptroller Thomas DiNapoli, who controls the pension funds of public workers from New York, made the announcement on Monday.
He said he was impressed by NI's "highly skilled workforce".
Deputy First Minister Mr McGuinness said it was a "massive vote of confidence" in the peace process.
While Mr McGuinness was in New York, First Minister Peter Robinson was holding talks with US firms which employ people in Northern Ireland.
The two leaders will meet up again in Washington, where they will meet US President Barack Obama on Tuesday at a St Patrick's Day reception at the White House.
Mr McGuinness said that, despite recent dissident murders, people in the US understood there was opposition to groups derailing the peace process.
"I have to say I am very encouraged by the outcome of our discussions," he said.
"We have been through a difficult week back home and it was very important to see if people were shaken by it here.
"It is clear that they are not."
However, SDLP leader Mark Durkan warned against people thinking that investment money from the United States comes free.
He said people should remember that interest rates in repaying the US loans can be higher than the banks.
"People need to understand that the Emerald Fund is pension fund money. We are talking about a serious commitment by them and they obviously have to be looking for a return," he said.
Robert Whalen of the New York Comptroller's Office told BBC Radio Ulster that expected returns from the investment were "very much on a case-by-case basis".
He said they had made the "long-term" investment "because there are great opportunities in Northern Ireland".
"Certainly it's not a handout - we have one million working New Yorkers who rely on the pension fund for their retirement - so we are looking for strong returns," he said.
"We think that we can get that, based upon the promise of opportunity that exists right now.
"It's a difficult market globally, but we think that over time there will be very strong returns for us."
He said it was too early to say whether the money would go into a number of large investments or a series of smaller ones.
The first and deputy first ministers, local politicians and Taoiseach Brian Cowen will be in Washington DC for a series of St Patrick's Day events hosted by President Barack Obama and Vice-President Joe Biden.