The Hawks led Thursday night's match by 17 points with just over seven minutes remaining before Fremantle piled on five unanswered goals to snatch the 12.16 (88) to 11.7 (73) win.
Fremantle racked up 21 inside 50m entries to four in a rampant final quarter, with ruckman Luke Jackson the key architect in a nine-disposal, three-clearance term.
The weight of numbers eventually took its toll as Hawthorn's players began to look increasingly helpless in the wake of the Fremantle avalanche.
Mitchell didn't sugar coat his assessment when analysing the game.
"We knew coming here to play ... that we were going to be up against it, and we were going to have to play 120 minutes," Mitchell said.
"Unfortunately, 90 won't get it done against the Dockers, and we learned that lesson. Hit us right in the face to be fair."
Mitchell conceded the territory battle became overwhelming in the final quarter.
"We went away from a few things that we were meant to be doing across that last 15 minutes of the game," Mitchell said.
"We already half-reviewed it, to be honest, about these are things that are going to cost us big games.
"We shot ourselves in the foot with the way we set the game up tonight, late in the game.
"We've talked a lot this year about being ready to play and win against the best sides, and we proved tonight that we're not quite there."
In an added blow, Hawthorn are set to be without star defender Tom Barrass for at least several games after the former Eagle injured his left hamstring in the second quarter.
Veteran forward Jack Gunston, who kicked five first-half goals on the way to a six-goal haul, injured his ankle late but was able to play out the game.
Hawthorn (6-2-1) will be aiming to bounce back from a winless fortnight when they face Melbourne at the MCG next week.
