Reed, who began the event 10 point ahead of Kevin Windham, worked his way forward from seventh position - passing the Honda rider for the lead on lap of 20
en route to his ninth victory of the season.
Red Bull Honda Racing's Andrew Short finished third.
"This really is a dream come true," said an emotional Reed, who won his first AMA Supercross title in 2004. "There have been a few bumps along the way but the ups and the downs are what you work hard for.
"At times this year I've thrown it on the ground, but been able to get back up on the podium, it's been an awesome learning year and I've learnt that every point counts.
"A lot of people questioned how hurt I really was [at Detroit]. Unfortunately, I spent the entire day at the hospital with the broken scapula. I had been there before [an injured shoulder] but this time I couldn't really get rid of the pain.
"You have to fight to the end. That's why it's a championship. That's why it's run over 17 rounds," Reed concluded.
After some well earned celebrations Reed's focus will shift to turning a long time dream into reality when he returns to Australia in early June to launch Super X, the Australasian Supercross Championship, in conjunction with promoters Rush Sports.
As well as having an active role in the running of the series Reed will also compete in the seven round national championship which debuts in Perth on October 4.
"I am really looking forward to heading home to launch Super X," Reed stated. "It has been a dream of mine for a very long time to turn Supercross into a mainstream sport in Australia.
"We have some really exciting things planned for both the fans and competitors and I don't think there is any other Australian domestic championship which can lay claim to having a world champion competing in the entire series.
"It is a great feeling to be able to give back to the sport that has given me an amazing career," Reed acknowledged.
Reed's Team San Manuel Yamaha mechanic, Oscar Wirdeman, was awarded a 2008 Toyota Tundra truck as the season's top mechanic.
