The McLaren junior led from start to finish in his DAMS machine, but he had to contend with pressure from Will Stevens (P1/Strakka) throughout the race to hang on for victory.
Magnussen appeared to have got the better of Stevens once and for all as the gap crept up towards three seconds mid-race, but he allowed the Briton to close up with an uncharacteristic mistake under braking at Turn 10.
After the race leader had locked up at the end of the backstraight his lead was down to one second, and Stevens was able to reduce that to 0.8 seconds at the start of the final lap.
But Magnussen hung on, giving him more than enough points to prevent Fortec's Stoffel Vandoorne from being able to take the title in Sunday's season finale.
Vandoorne finished a distant and lonely third in the race, having passed a slow-starting Will Buller off the line.
Buller's Zeta entry appeared set to take fourth, but after fifth-placed Sergey Sirotkin (ISR) retired with a broken rear wing, Buller came under pressure late on from Antonio Felix da Costa. The Briton eventually ran wide at Turn 10, giving the Red Bull junior the chance to pass to secure fourth for Arden Caterham.
Only 12 cars were running at the finish, with several of them being eliminated in a first lap accident triggered when Nigel Melker and Marco Sorensen came together at Turn 2 on the opening lap.
Their clash tipped Sorensen into a roll, and several other cars were then caught up in the melee that followed.
Results - 27 laps:
Pos Driver Car Time/Gap
1. Kevin Magnussen DAMS 46m32.366s
2. Will Stevens P1/Strakka +0.849s
3. Stoffel Vandoorne Fortec +11.582s
4. Antonio Felix da Costa Arden Caterham +23.899s
5. William Buller Zeta +29.431s
6. Zoel Amberg Pons +30.196s
7. Matias Laine P1/Strakka +33.363s
8. Daniil Move SMP/Comtec +37.209s
9. Pietro Fantin Arden Caterham +42.491s
10. Oliver Webb Fortec +43.543s
11. Lucas Foresti SMP/Comtec +46.306s
12. Nico Muller Draco +1m14.850s
13. Nikolay Martsenko Pons +2 laps
Mikhail Aleshin Tech 1 23 laps
Christopher Zanella ISR 23 laps
Sergey Sirotkin ISR 14 laps
Arthur Pic AV 12 laps
Andre Negrao Draco 9 laps
Carlos Sainz Jr Zeta 2 laps
Marco Sorensen Lotus 0 laps
Nigel Melker Tech 1 0 laps
Norman Nato DAMS 0 laps
Marlon Stockinger Lotus 0 laps
Yann Cunha AV 0 laps
Carlos Huertas Carlin 0 laps
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