An internal watchdog said Mr Comey had failed to "live up" to his duty to safeguard sensitive information.
The watchdog said Mr Comey broke FBI rules by giving a memo to a friend who shared its contents with a reporter.
But the inspector general said he was not recommending that Mr Comey be prosecuted for the breach.
What did the report say?
Mr Comey has been a vocal critic of Mr Trump since the president fired him in May 2017.
The report said that, after losing his job, Mr Comey broke the rules by divulging the unclassified parts of a memo he had written on his private conversations with Mr Trump.
He passed contents from the document to a friend and authorised him to share them with the New York Times.
The watchdog's report also found that Mr Comey violated departmental policies by keeping copies of the memos in a safe at his home and failing to notify the FBI that he had done so.
Why did he share the memo?
The memo alleged that Mr Trump asked Mr Comey to drop the FBI's inquiry into the president's National Security Adviser, Michael Flynn.
Mr Flynn was later fired for misleading the White House about his interactions with Russian officials. He pleaded guilty to lying to the FBI about the encounters.
The inspector general said Mr Comey shared the memo because he wanted to pressure the Department of Justice to launch an independent investigation into his private discussions with the president.
What was the reaction?
Despite the report's findings, Mr Comey claimed vindication.
The good news for former FBI Director James Comey in the inspector general report is the conclusion that he did not break the law and will not face criminal prosecution. The bad news is everything else.
Mr Comey violated FBI policies and standards. He, in effect, acted as though the rules didn't apply to him.
President Trump and his supporters will point to this as evidence that Mr Comey was a bad actor with an axe to grind against the president. Those in Hillary Clinton's circle will note, with irony, that the man whose investigation into the former secretary of state's handling of government documents that helped derail her presidential campaign is now on the receiving end of a not-prosecutable-but-not-proper finding.
Mr Comey's claim that the inquiry is a vindication is a real stretch. It is, rather, a stinging rebuke of the nation's former top cop - a man who in his writings and public statements has presented himself as striving for virtue in a toxic political world.