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.
"I worked my butt of this year," said Reed, who won his first AMA Supercross title in 2004. "I had a new bike this year, and that made my job a lot easier. It has been a great season for me this year. This is very special. It was unfortunate that James Stewart, the defending champion, got hurt, but this was a great year for us."
Reed's Team San Manuel Yamaha mechanic, Oscar Wirdeman, was awarded a 2008 Toyota Tundra truck as the season's top mechanic.
In the Dave Coombs Sr. East/West Supercross Lites Shootout, Rockstar Makita Suzuki's Ryan Dungey followed Progressive Holeshot Award winner Ryan Villopoto's Monster Energy Pro Circuit Kawasaki through turn one.
It quickly became a two-horse race as the two 2008 title runner-ups pulled away from the field. Villopoto led the first 14 laps while Dungey inched in Villopoto's lead. On the last lap, Dungey, who won last year's Dave Coombs Sr. East/West Shootout, passed Villopoto and rode to victory.
Dungey was awarded a brand new 2008 Toyota Tundra truck for winning the Dave Coombs Sr. East/West Shootout. Newly crowned East coast Lites champion Trey Canard finished third.
"To win this race two years in a row is huge for me," said Dungey. "It was a very special win for me because I came from behind and rode smart. I lost some time in the beginning of the race, and I was able to find my grove as the race progressed."
"That was one of the fastest starts I have ever raced on at a Supercross race," said Villopoto. "The lappers played a big part in the race, but Ryan [Dungey] was riding great. I made some mistakes, and he capitalised on them."
Atomic Racing's Paul Carpenter won the U.S. Smokeless Tobacco Pro Privateer Challenge (USSTC) and was awarded $25,000.
AMA Supercross Class Results, Las Vegas
1. Chad Reed, Tampa, Fla., Yamaha
2. Kevin Windham, Centreville, Miss., Honda
3. Andrew Short, Smithville, Texas, Honda
4. Josh Hill, Yoncalla, Ore., Yamaha
5. Davi Millsaps, Cairo, Ga., Honda
6. Nick Wey, Dewitt, Mich., KTM
7. Josh Summey, Huntersville, N.C., Yamaha
8. Travis Preston, Hesperia, Calif., Kawasaki
9. Tim Ferry, Dade City, Fla., Kawasaki
10. David Vuillemin, Murrieta, Calif., Suzuki
AMA Supercross Class Season Standings
1. Chad Reed, Tampa, Fla., Yamaha, 365
2. Kevin Windham, Centreville, Miss., Honda, 352
3. Andrew Short, Smithville, Texas, Honda, 281
4. Davi Millsaps, Cairo, Ga., Honda, 278
5. Joshua Hill, Yoncalla, Ore., Yamaha, 228
6. Tim Ferry, Largo, Fla., Kawasaki, 218
7. Nathan Ramsey, Menifee, Calif., Yamaha, 173
8. David Vuillemin, Murrieta, Calif., Suzuki, 169
9. Nick Wey, Dewitt, Mich., KTM, 159
10. Paul Carpenter, Ithaca, N.Y., Honda, 145
Dave Coombs Sr. East/West Shootout
1. Ryan Dungey, Belle Plaine, Minn., Suzuki
2. Ryan Villopoto, Poulsbo, Wash., Kawasaki
3. Trey Canard, Elk City, Okla., Honda
4. Dan Reardon, Murrieta, Calif., Honda
5. Ryan Morais, Murrieta, Calif., Yamaha
6. Justin Brayton, Murrieta, Calif., KTM
7. Brett Metcalfe, Murrieta, Calif., Kawasaki
8. Kyle Chisholm, Seminole, Calif., Kawasaki
9. Broc Hepler, Kittanning, Pa., Yamaha
10. Austin Stroupe, Corona, Calif., Kawasaki