Prosecutors made the agreement with the key cooperating witness public after a federal judge in Brooklyn, New York, on Thursday ordered it unsealed at the request of media outlets following the indictment of nine current and former FIFA officials and five corporate executives.
The document revealed that Blazer, the former general secretary of CONCACAF, soccer's governing body in North and Central America and the Caribbean, began providing prosecutors information as early as December 2011.
Under the agreement, Blazer agreed to provide prosecutors information, turn over any documents he possessed related to the probe, participate in undercover activities and testify at trial.
The Justice Department had initially opposed making the agreement public, arguing that confirming that Blazer was a cooperating witness would prejudice its investigation and jeopardize his safety.
But US District Judge Raymond Dearie on Thursday rejected those arguments, saying prosecutors had failed to meet their "high burden" to establish the document should remain sealed.
In a ruling released on Monday explaining his reasoning, Dearie noted the press had already extensively reported on Blazer's cooperation, including that it began in 2011 and involved recording conversations.
"The nature of Blazer's cooperation - at least to the extent expressed in his cooperation agreement - should be removed from the shadows," Dearie wrote.
A spokeswoman for the Brooklyn U.S. Attorney's office declined comment. Blazer's lawyer, Eric Corngold, declined comment.
Blazer, 70, is one of four individuals who had secretly pleaded guilty in the years before the case was announced on May 27.
He has become a key cooperating witness in the probe, which has engulfed FIFA and led the governing body's president Sepp Blatter to announce his resignation.
Blazer, who had been a FIFA executive committee member from 1997 to 2013, secretly pleaded guilty in November 2013 to 10 counts including conspiracies to commit racketeering, wire fraud and money laundering.
According to a redacted transcript of his plea hearing, Blazer admitted that he and other officials took bribes in connection with the 1998 and 2010 World Cups.
Blazer also admitted to with others accepting bribes and kickbacks related to five different editions of CONCACAF's premier event, the Gold Cup, between 1996 and 2003.
He also admitted to committing tax evasion and to working with others to transfer the money between accounts in an effort to conceal the kickback schemes.