Riding his first race since fracturing his collarbone, sternum, and scapula in a crash at the Tour of the Basque Country in April, Cummings entered the race in unknown form, but pulled out a stunning ride to beat Dowsett by just eight seconds on the 44.4km course.
Setting off in the first half of the 44-rider field, Cummings did not have any splits to aim for on his ride, and set a time of 28-42.48 through the first of the two 22.2km laps.
That time stood for nearly three quarters of an hour until Dowsett, who as defending champion had been the final man down the start ramp, went 13 seconds faster.
Almost at the exact moment that Dowsett went through the first lap, Cummings was completing his ride, setting a time of 57-18.66 to put him in the hot seat.
For a while no rider was able to get close to that mark, with Tao Geoghegan Hart (Team Sky) and Harry Tanfield (Bike Channel-Canyon) both finishing around 1-30, before James Gullen (JLT Condor) went into provisional second with a time of 58-09.45.
From there it was going to be between Cummings and Dowsett, and with the advantage from the earlier split, the odds seemed to be in the favour of the Movistar man who was chasing a sixth national time trial to put him level with the record held by Stuart Dangerfield.
However it was not to be for the defending champion, who wasn’t able to hold onto his lead, eventually finishing eight seconds down to give Cummings his first national title on the road since he took the junior road race in 1999.
Earlier in the day Scott Davies (Team Wiggins) won a fourth consecutive under-23 title ahead of Tom Baylis (One Pro Cycling) and Charlie Tanfield (Brother NRG).
Men’s British National Time Trial Championships (44.4km)
1. Steve Cummings (GBr) Dimension Data, 57-18.66
2. Alex Dowsett (GBr) Movistar, at 8 secs
3. James Gullen (GBr) JLT Condor, at 50 secs
4. Tao Geoghegan Hart (GBr) Team Sky, at 1-27
5. Harry Tanfield (GBr) Bike Channel-Canyon, at 1-33
6. Peter Williams (GBr) One Pro Cycling, at 1-52
7. Ryan Perry (GBr) Raleigh GAC, at 1-55
8. Sam Harrison (GBr) Team Wiggins, at 2-30
9. Richard Handley (GBr) Madison Genesis, at 2-47
10. Jon Dibben (GBr) Team Sky, at 3-18