Adam Schiff, the top Democrat on the House of Representatives Intelligence Committee, which is also investigating allegations of Russian collusion, said the memo indicated Flynn had provided a significant boost to Mueller's probe.
"That most of the details are redacted signals he has given far more than we or the President may know," Schiff said on Twitter.
Flynn, a retired army general, was forced to resign as national security adviser in February 2017 after he was found to have misled Vice President Mike Pence about discussions he had with Russia's then-ambassador, Sergei Kislyak.
Under a plea bargain deal, Flynn admitted in a Washington court that he lied when asked by FBI investigators about conversations with Kislyak just weeks before Trump took office.
Prosecutors said the two men discussed U.S. sanctions against Russia and that Flynn also asked Kislyak to help delay a United Nations vote seen as damaging to Israel.
Flynn's crime of lying to the FBI carries a statutory maximum sentence of five years in prison. His plea agreement states, however, that he is eligible for a sentence of zero to six months and can ask the court not to impose a fine.
Mueller's office has had varied degrees of success with the level of cooperation it has received from defendants who have pleaded guilty.
Cohen has also cooperated with the probe. He is due to be sentenced on Dec. 12 after pleading guilty to charges brought by prosecutors in New York including tax evasion and campaign finance violations. His lawyer has asked the court to consolidate the sentencings for that case and the recent one brought by Mueller's office.
Manafort, in contrast, breached his plea deal by lying repeatedly to the FBI and Mueller's team, prosecutors alleged in a court filing last week.
Mueller is due to file court papers on Friday laying out his team's case for why Manafort should lose any credit when he is sentenced because of his alleged failure to accept responsibility for his crimes.
David Berger, a partner with Wilson Sonsini Goodrich & Rosati, said Mueller was sending an important message to Cohen and other cooperators by supporting Flynn and coming down hard on Manafort, who could now face the rest of his life in prison.
In addition to his cooperation with various probes, Mueller noted Flynn's record of military and public service and argued that it distinguished him "from every other person who has been charged" as part of his investigation. "Michael Cohen wants to serve no prison time. He has now been given a road map," Berger said.
