The incredible feat was achieved after nine months of preparation and rivalry to fellow biker James Foster.
Mildon, 27, pipped him to the record after a number of - what looked to be pretty painful - aborted attempts from both men.
He said: "This is the pinnacle of my life. Kinda like a story that you want growing up as a child but you don't know how it's going to pan out. It's unreal.
"To finally stomp the quad and reach my goal is unbelievable. I'm so pumped.
"It wouldn't have been possible without the support of my family, including my Nitro Circus mates.
"James Foster is such an incredible athlete and a great friend. He pushed me to challenge myself and test my limits."
Mildon was also the first BMXer to land a triple flip back in 2011 which you can watch below...