The world number 11 had defeated Gary Anderson, Michael van Gerwen and Jose de Sousa to move himself to within a match of making history, and the 32-year-old had the better of the final against Chisnall.
While Chisnall won three legs in a row to make it 5-5, Cullen surged clear again, and a couple of crucial checkouts put himself to within one leg of victory at 10-7.
However, that is when nerves started to set in, Cullen missing four match darts in the 18th leg before a further six were wasted in the following leg.
At the same time, however, Chisnall was also wilting, and it allowed Cullen to confidently hit double 18 to seal the biggest triumph of his career.
The Bradford thrower will now hope to win a wildcard for the Premier League, with the eight-player lineup being announced on Monday morning. body check tags ::