Will Hayward sparked the Swans with the first majorl at the MCG before the Lions piled on seven goals to one in the second term to steamroll towards to a 60-point victory.
Sydney looked to mount a second-half challenge, as they have done repeatedly this season, but they were disjointed across all lines.
Under Brisbane heat, the Swans gave away 62 turnovers to 53 and kicked just nine majors from 49 inside-50 entries.
In comparison, the Lions kicked 18 goals from 51 inside-50 entries.
Losing their clutch hero Isaac Heeney to an ankle injury in the third term only weakened their resolve.
Swans utility Fox, one of the few shining lights in John Longmire's team, admits the game was already lost between the ears before the final siren sounded.
Fox finished with 21 touches, one goal and two clearances, while midfielder James Rowbottom battled desperately in Heeney's absence with 18 disposals and 10 clearances.
James Rowbottom slams home a goal on a disappointing day for the Swans at the MCG. (James Ross/AAP PHOTOS)
"It was physical to start with. They came for the fight and we weren't up to it," Fox said.
"Then it turned into a mental game and you just have to be off one or two per cent.
"It's just disappointing to not show up after the second quarter pretty much.
"It's funny to say we had a great feeling before the game. Everyone was calm and composed."
Their defeat comes two years after their 81-point grand final thrashing by Geelong.
It is also the club's fourth loss in a decider since the 2012 premiership, leaving Longmire with a 1-4 record on the last Saturday in September.
Fox says knowing the Swans were primed to win the flag after dominating the regular season to claim the minor premiership only compounds their devastation.
"We had such a good year to put ourselves in this position and then it's just that cherry on top," Fox said.
"We were a little bit more in it this time around.
"To put ourselves in such a good position, finishing top, having the week off and getting back here, it does hurt just to not show up again."
"I obviously couldn't get through and in the end, it was too far a deficit and I wasn't moving well."— AFL (@AFL) Isaac Heeney opened up on the injury he'd been carrying throughout the finals series. https://t.co/0wP2hTIRVYSeptember 29, 2024
Having learnt how to regroup after their brutal hammering from the Cats two years ago, Fox says the team will be trying to bottle the positives from their season.
"I spoke to Isaac after the game, he was very devastated but I was like, 'You've had such an amazing year, you helped get this team here'," Fox said.
"Throughout the finals series, we did a lot of work with (sports psychologist) Emma Murray and we were like one game won't define us.
"This was four weeks ago and we just had this mentality.
"We would like to win a grand final, but it won't define us.
"It wasn't this year, but hopefully it's in the years to come."
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