Mattocks scored in the 84th minute — just a minute after being put on the pitch — as the Whitecaps earned a 1-1 draw against the rival Portland Timbers.
Vancouver manager Marin Rennie put Eric Hassli in in the 72nd minute, hoping to jumpstart an offence that slowed dramatically after an early second half flurry.
Mattocks followed Hassli on to the pitch 11 minutes later and the two forwards connected to score the key goal.
"Hats off to coach Rennie for putting me and Eric in together at the end of the game," said Mattocks. "(Hassli) is the big guy who holds up the ball and I'm the short, speedy guy who finishes those combinations."
Mattocks played down a Hassli header, eluded a Portland defender and emphatically shot with his right foot. The rookie's goal was his first in Major League Soccer, securing a valuable draw on the road.
"I really believe we deserved that goal," said Rennie. "(Mattocks) is always going to be creating chances for himself because of the way he plays.
"If he can be taking those chances, we've got a really special player on our hands."
Neither team could find a goal in an even first half defined by midfield play.
Just before half-time, the weather turned suddenly, dropping temperatures amid ferocious wind and rain.
Vancouver nearly opened scoring in the 45th minute when Sebastian Le Toux fed Camilo in the six-yard box. The Brazilian forward poked his shot just wide to the right — the Whitecaps' only shot of the half — and the teams hurried to the dressing rooms.
Lightning delayed the start of the second half for 25 minutes, but it did nothing to deter the Whitecaps patient, attacking style.
As the weather returned to normal, Vancouver avoided disaster when Darlington Nagbe's shot slipped just over the bar in the 50th minute. Le Toux forced a reaction save from Troy Perkins in the 58th minute.
Whitecaps 'keeper Joe Cannon made two diving saves in two minutes to deny Portland midway through the second half but could not come up with another spectacular play to keep Kris Boyd from scoring for Portland in the 68th minute.
The Timbers' designated player volleyed in a rebound after Cannon failed to gather a quick shot from inside the box.
Camilo nearly levelled the score for Vancouver with a skipping shot in the 74th minute.
After playing five games in two weeks, Vancouver does not play again until hosting Houston Dynamo on June 10.
Rennie described the draw in Portland as the end of the first stage of the 2012 season — a stage that leaves the Whitecaps in fifth place in the Western Conference on nineteen points.