And he promised to be closer to 170lb by the time he enters the ring at T-Mobile Arena tomorrow.
McGregor weighed in at 153lbs, one pound under the limit of 154 with Mayweather weighing 149.5.
But the Irishman came in safely inside the limit and will now try and do what no other man has done - beat the 49-0 ring legend.
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Although why he would want to use the banned procedure to rehydrate before the weigh-in was a puzzler from the Money Man.
Pretty Boy said his rival was struggling to make the 154lb mark as he walks around Dublin at around 168lb.
But the fewer weight classes in the UFC mean the MMA master is well versed in boiling down at will.
The 29-year-old looked gaunt and strained when he made 145lb mark to face Jose Aldo in 2015.
But he looked rejuvenated when he entered the ring to face the feared Brazilian and took just 13 seconds to do his work.
After the first Nate Diaz fight, when McGregor suffered a loss and appeared to blow out early, he pointed to being too heavy and bulking up too dramatically not being fit enough.
For the rematch he looked leaner and made no mistake, going the full 25 minutes and getting his revenge.
Mayweather was hoping McGregor misjudges the weight cut this week and lost mobility and movement before struggling over a possible 36 minutes.
But the Irishman did his training camp at the new UFC performance institute where he had the sports science equipment and nutritionists at his disposal and got it spot-on.
And, on Thursday, Mystic Mac’s nutritionist revealed his man was cruising toward his 11 stone target.
George Lockhart said: "It's perfect. I'd even call it a masterpiece.
“He woke up this morning eight pounds overweight. He'll get down to maybe 156 pounds by tonight.
McGregor emerged first. Topless and draped in his beloved Irish tricolour.
McGreogor threw a few punches and bobbed and weaved, looking well under the 154lb mark he was soon to prove to be.
A quick hug with UFC boss Dana White before acknowledging the legions of fans that had finally arrived to support him.
Mayweather's entrance was far less dramatic. He showed his age with middle-aged crooner Phil Collins as his entrance music.
He skipped around the stage, purposely it appeared trying to glad McGregor who almost bit at the blatant provocation.
McGregor made it at a 1lb under and roared while bulging every sinew of his dehydrated frame and after roaring away he began chugging water.
Mayweather - with his concerned father dressed like a Christmas tree on his shoulder - weighed in five pounds under at 149lb.
McGregor screamed in Money's face for what seemed a cruel and childish eternity and ended his tirade with "let's do it b***h".