The 70-year-old struck a deal to become an informant to avoid a jail term of up to 75 years, pleading guilty to ten charges, including bribery, money laundering and tax evasion.
His plea agreement has been made public and confirms he agreed to work undercover from December 2011.
The disclosure was revealed as Fifa expressed angry disappointment at the Nobel Peace Centre’s decision to sever its links with the world governing body.
Blazer and three others have pleaded guilty to football-related corruption, while another 14 people have been indicted. These include seven Fifa officials arrested in Switzerland on 27 May, who are contesting extradition to the United States.
Blazer’s 19-page plea agreement says: “The defendant agrees to furnish to the office all documents and other material that may be relevant to this investigation... and to participate in undercover activities pursuant to the specific instructions of law enforcement agents.”
The agreement does not detail what Blazer’s sentence will be but says his co-operation with the authorities can be taken into account.
Blazer, a Fifa executive committee member from 1997 to 2013, has admitted taking bribes to vote for South Africa to host the 2010 World Cup, and named former Fifa vice-president Jack Warner as also taking bribes.
Warner was president of north and central American and Caribbean confederation CONCACAF and Blazer his general secretary. Blazer admitted, among a series of revelations, that he and Warner took bribes from Morocco for its 1998 World Cup bid and from South Africa for 2010.
The plea agreement disclosed Blazer, who is seriously ill with cancer, has already forfeited $1.95 million as part of his illegal proceeds and will face making a second payment when he is sentenced. He also admitted to accepting bribes in relation to TV and marketing rights for the CONCACAF Gold Cup.
It was reported last year that Blazer had bugged meetings with executives at the London 2012 Olympics with a wire device concealed in a key fob.
Meanwhile, Fifa president Sepp Blatter’s hopes of winning a Nobel peace prize appeared to have disappeared after the centre broke off its relationship with Fifa.
Fifa said it had only learned of the move via the media and that the action was against “fair play”.
A Fifa statement said: “We are disappointed to have learned from the media about the Nobel Peace Centre’s intent to terminate the cooperation with Fifa on the Handshake for Peace initiative.”