Nonetheless, the result is a marvellous one for the 27-year-old, while Whincup joins Allan Moffat and Greg Murphy as four-time winners at Mount Panorama.
The victory is Dumbrell's first in his 12th start in the great race.
Holden Racing Team's Courtney came home fourth in his best result of the season while Caruso finished fifth.
At the other end of the field, another championship contender, Ford Performance Racing's Will Davison, started on pole and was leading in the early stages but endured a horror day.
He and co-driver John McIntyre had to settle for a 24th place finish after Davison was lucky to survive when his brakes failed going into Conrod Straight and he was sent spearing across the track at over 250km/h, narrowly missing a passing car on the track.
Davison's teammate Mark Winterbottom came in 11th after failing to get himself up to the front throughout the race and has relinquished second place in the championship to Lowndes.
Defending champions Garth Tander and Nick Percat also had a shocker, their title defence effectively ending in the 38th lap when their Holden Racing Team Commodore was damaged following a collision with a wall at the top of the mountain.
Percat was behind the wheel when it appeared to suffer a mechanical problem and despite the pair getting back on the track, the time lost to repair the vehicle left them out of contention.
-AAP