The champion Geelong player argued that contact with Brisbane's Pearce Hanley was "unintentional and unavoidable".
"As I got closer I intended to tackle," Johnson told the Tribunal.
"(Joel) Corey surprised me by making the tackle and I tried to brace myself knowing I was on a collision course with him."
The Lions had earlier given a medical report stating that no injuries were sustained, no treatment was required, and no weeks would be missed by Hanley.
But after six minutes of deliberation, the jury came back with a guilty verdict, meaning Johnson will miss matches against Fremantle and Hawthorn.
