Daniel Keneally, 25, was convicted in the Downing Centre District Court on Tuesday.
Keneally wrote a statement containing many falsehoods and rejected his lawyer's claims that it was an honest mistake, Magistrate Rodney Brender found.
The offence related to an incident where Keneally was a few hours into a night shift at Newtown police station when Luke Brett Moore called in February 2021.
Keneally's written statement, which he told the court he felt pressured to make after talking to higher-ranking officers, resulted in Mr Moore being arrested and held in custody for three weeks.
The activist and founder of ISuepolice was later released on bail and the charge dropped, due to a recording of the conversation made on his phone.
The recording, taken without Keneally's knowledge, showed significant discrepancies to what was in the officer's statement.
These included Keneally naming a police officer he alleged Mr Moore threatened to kill, which had not occurred.
Magistrate Brender said the conversation as recorded by Keneally in his statement "just didn't happen".
"There is no basis to suggest any other person suggested the content or affected (Keneally's) memory," he said.
The magistrate found Keneally knew it was a real possibility that evidence of a threat to a policeman could lead to a prosecution.
Keneally also knew his statement could be used in any court proceedings and would mislead relevant tribunals, the magistrate said.
Keneally will remain on bail and be sentenced on December 21.
Outside court, Keneally's lawyer Paul McGirr said his client "holds his head high".
"We respect the court's decision but can say we will definitely appeal on the basis that my client never wilfully put something in a statement that he knew to be false," he said.
"It's a very high hurdle for the prosecution to prove."
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