At its biggest, their lead was about 8.1 seconds but Reynolds and Canto, who both achieved their first-ever podium finish in Supercar racing, dogged them for more than half the race.
Whincup joined Australia's Allan Moffatt and New Zealander Greg Murphy as a four-time winner of the race while Dumbrell was a winner for the first time in 12 starts at Mount Panorama.
"What a phenomenal day," Dumbrell said. "You could not get a better feeling than what we're feeling right now."
Reynolds pressed Whincup as hard as was possible on wearing tyres through the last laps of the race but was unable to find the pace or space to pass.
The marathon race provided its usual high rate of attrition, even among the leading drivers. Defending champions Garth Tander and Nick Percat saw their defence effectively end on the 38th lap when, with Percat at the wheel, a mechanical failure caused their car to glance a wall.
Will Davison and John McIntyre, who started from pole position in a Ford Falcon, finished 24th after a brake failure caused them to spear off the track at more than 250km/h.
The 50th anniversary edition of the race was attended by a record crowd of 207,205 over its four days.