The new Mueller memo says Mr Flynn had provided assistance in multiple investigations and had also given information on co-ordination between the Russian government and the Trump campaign.
Flynn, whose sentencing is scheduled for Dec. 18, pleaded guilty in December 2017 to lying to the Federal Bureau of Investigation about his conversations with then-Russian Ambassador Sergey Kislyak during the presidential transition period from election day 2016 to Trump's inauguration in January 2017.
Prosecutors also say Flynn's "record of military and public service" sets him apart "from every other person who has been charged" in Mueller's investigation.
Flynn's sentencing is scheduled for December 18. The sentencing guidelines for Flynn's offense suggest 0 to 6 months of incarceration.
Mueller was appointed to his role in May 2017, directed by deputy attorney general Rod Rosenstein to investigate "any links and/or coordination between the Russian government and individuals associated with" President Donald Trump's presidential campaign. It adds that Flynn has met with the investigators 19 times, and that Mueller is extremely satisfied with Flynn's cooperation.
Flynn was a close adviser to Trump during the campaign - and famously led a "lock her up" Hillary Clinton chant at the Republican National Convention. Not only did he draw notice to how soon into the investigation he agreed to cooperate, but he also pointed out how valuable his cooperation has been. In addition, Mueller's office announced former Trump campaign chairman Paul Manafort violated terms of his plea agreement.
According to memos written by former FBI Director James Comey, Trump tried to protect Flynn by asking Comey to let the investigation into his false statements go.
Trump has tweeted in the past in support of Flynn, saying that Mueller had "totally destroyed" his life unfairly. One knowledgeable source said Flynn's cooperation largely took place immediately following his December 2017 court appearance.
Other than Flynn and Cohen, Mueller's team charged former Trump campaign foreign policy adviser George Papadopoulos with making false statements to the Federal Bureau of Investigation.
The filing is the first time prosecutors have described Flynn's assistance since his guilty plea a year ago. "It is a shame because his actions during the transition were lawful", Trump tweeted after Flynn pleaded guilty a year ago.