He returned 45 minutes later and made weight without having to remove his underpants, much to the relief and then encouragement of the Australian supporters
He risked giving up his world title belt had he not made weight.
When he stepped on the scales the first time, Horn was watched closely by Terence Crawford's trainer, Brian "Bomac" McIntyre.
And when Horn missed the weight, McIntyre and Crawford reacted predictably by laughing.
Horn was surprised to have missed weight and said he'd come in under weight when he tried it on scales in his hotel room.
"We tried it on the main scales in the room — obviously they've got it wrong up there because we were way under on our scales," Horn said.
When asked if he thought he would make weight, Horn was adamant.
"Yes, definitely. I thought I was going to be probably underweight. Obviously the scales are a bit tricky — oh well, I've only got half a pound to lose."