The Dockers kicked the first four goals of the match and led at every change before the Kangaroos ran over the top of last year's grand finalists in the last quarter to win by 13 points.
Johnson was exasperated at the skill errors that cost the Dockers victory and left them teetering at 3-3 after six rounds.
"It's a game that got away from us," Johnson said.
"We brought effort and we tried really hard.
"But what hurt us was the turnovers, and they were able to score on our turnovers.
"It's a hard one. It's one that got away."
Fremantle won all of the major statistical categories including contested possessions, clearances, and inside 50s but it butchered the ball throughout the night just as it did against the Sydney Swans in round five.
"It just comes down to simple errors," Johnson said.
"Easy dropped marks, missing the easy target and just not doing the things we've trained and worked on over the last couple of years. It's cost us today and also last week.
"We've just got to work on the skill execution, especially the midfield."
The Dockers will have to rebound quickly as they prepare to face West Coast in what may be a pivotal Western Derby which could have significant ramifications for both team's finals aspirations.
Johnson said the side would review its performance against North Melbourne thoroughly in order to prepare for the Eagles.
"We've got to get back on the horse and come together in the meetings," Johnson said.
"It's a game that we're not going to push aside. We're going to have these meetings and go through them over the next few days. We'll work on the things that let us down."
