Johnson was offered a one-match suspension by the Match Review Panel for kneeing Thompson, but the Cats elected to challenge his charge, risking an extra week on the sidelines.
The gamble paid off for the Cats, who successfully argued there was insufficient force in the blow to warrant a charge.
Thompson was called by the Cats to give evidence, telling the Tribunal he had suffered broken ribs in round 18 and wore an AFL-approved guard to protect the injury.
Represented by Peter Murdoch QC, Geelong argued Thompson's reaction after contact was made to his ribs was due to the existing injury – which Johnson was not aware of – rather than the force of the blow.
The jury of Wayne Henwood, David Neitz and Wayne Schimmelbusch took 10 minutes to deem Johnson's behaviour was unsportsmanlike and unacceptable, and it would have warranted a misconduct charge if the force was sufficient.
However, the lack of force means he is free to face Fremantle in a crunch game for the Cats and Dockers.
Johnson will still carry 75.31 carryover points for head-butting Fremantle tagger Ryan Crowley earlier this season.
"I'm relieved by the result, I got a fair hearing, and I think the correct decision was made," Johnson said on Tuesday night.
"We found out [about Thompson's injury] later, but I'm glad he came in … it helped my case, no doubt."
A North Melbourne medical report said Thompson had not required any treatment because of injury and it was anticipated he would be available for Saturday's clash against Greater Western Sydney.
Thompson told the Tribunal the area Johnson made contact to with his knee was "pretty sensitive" and Johnson had accused him of faking pain once the pair stop wrestling.
Thompson said he didn't want to reveal his injury at the time because "it would be targeted".
Johnson told the Tribunal he was surprised by Thompson's reaction at the time.
Umpire Brett Rosebury also gave evidence, saying he did not view the incident constituted a report or a free kick at the time".
On Monday afternoon, the Match Review Panel assessed Johnson's blow to Thompson as reckless conduct, low impact and body contact.
Johnson has been suspended for six matches in the past three seasons.
