"LIED! LIED" and was "totally controlled" by Comey, whom Trump fired last May as the FBI was investigating possible collusion between the Trump campaign and the Russians.
His lawyer said the inspector general unfairly tried to conclude its work before McCabe could retire with a full pension.
Bromwich also fired off a lawsuit threat in light of Trump's latest outburst. He sarcastically thanked Trump in a Twitter post on Friday for "providing even more material" for the lawsuit he is considering. "Stay tuned", Bromwich tweeted.
"James Comey is a proven LEAKER & LIAR".
"President Trump will no doubt gloat about these findings and misuse them in his ongoing disinformation campaign", he continued, asserting that House Republicans "will nearly certainly try to use this report to fuel their efforts to distract from the abuses of power by this Administration and to undermine the Department of Justice".
In addition to Comey, Trump is pushing back against claims that he had extramarital affairs with porn star Stormy Daniels and Playboy playmate Karen McDougal.
The Associated Press has also confirmed that McCabe kept personal memos detailing interactions with the president and they have been provided to the special counsel's office.
The president has made a concerted and Twitter-driven effort to impugn Mr McCabe as a partisan hack, accusing him of covering up unspecified "lies and corruption" at the Federal Bureau of Investigation and calling his firing a "great day for Democracy".
President Donald Trump had accused McCabe of bias, and the president's critics said McCabe was being fired as part of a Republican effort to discredit a federal probe into alleged ties between Russian Federation and the Trump presidential campaign.
McCabe was sacked just days before he would have been eligible for a lifetime pension after it was determined that he misled investigators reviewing the bureau's probe of Hillary Clinton's email server.
The agency inspector general's report outlines an inquiry into Mr McCabe which US Attorney General Jeff Sessions cited when he fired him last month.
McCabe has also said he believes he was singled out because of the "role I played, the actions I took, and the events I witnessed in the aftermath" of Comey's firing.
A spokesman for the inspector general's office declined to comment.