Team Solitaire's Bobby Kiniry won the Progressive Holeshot Award with Dungey in second while Lawrence, who came into the race with a six-point lead over Dungey, in fourth position.
While Dungey pulled away from the field, Lawrence moved into second place and rode to the first AMA Supercross Lites championship of his career. Yamaha's Broc Hepler finished in third place.
"Once I settled into second place, I was really nervous," said Lawrence. "I just wanted to keep it on two wheels and ride a smart race. I was really comfortable when my Yamaha team-mate Broc Hepler was behind me."
The win was Dungey's third of the season, but mistakes in previous rounds cost him the crown.
"I was really happy with the way I rode tonight," said Dungey. "I know where I made some mistakes this year, and I am going to fix them. I learned a lot this year."
AMA Supercross Class Results, Seattle
1. Kevin Windham, Centreville, Miss., Honda
2. Chad Reed, Tampa, Fla., Yamaha
3. Andrew Short, Smithville, Texas, Honda
4. Davi Millsaps, Cairo, Ga., Honda
5. Tim Ferry, Largo, Fla., Kawasaki
6. Josh Grant, Riverside, Calif., Honda
7. Josh Hill, Yoncalla, Ore., Yamaha
8. Nick Wey, Dewitt, Mich., KTM
9. David Vuillemin, Murrieta, Calif., Suziki
10. Heath Voss, Mico, Texas, Honda
AMA Supercross Class Season Standings
1. Chad Reed, Tampa, Fla., Yamaha, 340
2. Kevin Windham, Centreville, Miss., Honda, 330
3. Davi Millsaps, Cairo, Ga., Honda, 262
4. Andrew Short, Smithville, Texas, Honda, 261
5. Joshua Hill, Yoncalla, Ore., Yamaha, 210
6. Tim Ferry, Largo, Fla., Kawasaki, 206
7. Nathan Ramsey, Menifee, Calif., Yamaha, 173
8. David Vuillemin, Murrieta, Calif., Suzuki, 158
9. Nick Wey, Dewitt, Mich., KTM, 144
10. Paul Carpenter, Ithaca, N.Y., Honda, 136
