For the David Bell-trained gelded son of Stormy Atlantic, the six-furlong event was his fourth victory of the 2008 campaign and first added-money triumph from 13 starts.
Main Executive broke sharply from post seven and was immediately challenged by Rouse the Cat, with the duo taking the field through early splits of :22.26 and :44.65.
Storm Treasure, third in this year's Breeders' Cup Turf Sprint, rallied stoutly in the late going, finishing a nose behind Rouse the Cat for the runner-up spot and a head in front of a hard-charging Field Commission.
The final time was 1:09.08 on the Polytrack.
For Stein, who has held a hot hand over the past few weeks, the key to victory was simply finding running room.
"He almost ran over the two horses in front of him," said Stein, of Shadowless, who finished 11th to Not Bourbon in this year's Queen's Plate. "I just had to wait a little longer. I pulled the trigger and away he went."
Bred and owned by Heather Takahashi, the dark bay came into the Kennedy Road off a third in the Labeeb Stakes on October 26, a race contested on the Toronto oval's E.P. Taylor Turf Course. On October 8, he finished second in the Mt. Sassafras Stakes.
Takahashi was especially pleased with the Ontario-bred's gutsy performance.
"It's an absolute thrill of a lifetime, a dream come true," she said. "I'm pinching myself. It's fantastic."
Shadowless paid $35.60, $17.60 and $9.70 across the board, combining with Rouse the Cat ($10.50, $6.20) for a $408.80 exactor. Storm Treasure ($4.10) rounded out a $2,084.20 triactor.