As for why the card is so coveted ... it's an ULTRA rare 2017 Panini National Treasures piece that features Mahomes' signature and a cutout of an NFL shield patch from one of his Chiefs jerseys.
It's the only card of its kind to ever be created (AKA a 1-of-1) ... and it was also given an 8.5 grade, which means it's in damn-near perfect condition.
The $4.3 million price tag easily topped the previous football card record of $3.1 million that an autographed Tom Brady rookie card fetched in the spring.
As for the buyer of the Mahomes piece, it's been revealed LJ's Card Shop in Ohio scored the card ... and a store spokesperson said they're fired up over the purchase.
“We are continually hunting for unique cards that add to our collection in a meaningful way," the spokesperson said. "The 2017 National Treasures NFL Shield Patrick Mahomes Rookie is truly a piece of art."
"We are honored to add this card to our collection.”