The event finished in fine fashion with a tube-riding shootout between two of the sport's greatest champions on 3-5ft waves.
Slater and Parkinson opened their 40-minute final bout with a rapid-fire exchange of tube-riding displays, vaulting the lead back and forth before Slater nailed a 9.93 for an incredibly-deep barrel punctuated by a full-velocity forehand gaff.
"It was just fun today," Slater said. "I really wanted to surf Kirra as I made known the other day and it was great fun out there.
"We thought the swell was going to be too East, but it's really really good out there and to get to surf it with Joel is even better. He's one of the best and we had a lot of fun out there."
The win is the 52nd top-level victory of Slater's amazing career, but the iconic Floridian admits there is a long season ahead in terms of the hunt for the 2013 ASP World Title.
"They're all important but it's nice to get off to a good start," Slater said. "The three top seeds – Joel, myself and Mick all finished in the top three spots and Michel (Bourez) had a great event too.
"Momentum shifts throughout each season and a lot can happen. It's probably the most talented field that has ever been on tour and there are nine more events. I'm committed to the tour this season."
Parkinson had looked unbeatable on the final day of the event, collecting a perfect 10 in his morning semi-final before posting an excellent 17.47 in the final against Slater.
But the impressive score would prove insufficient against the American's onslaught.
"When Kirra is doing what it's doing out there today, it doesn't feel like an event," Parkinson said.
"I had some great waves and a lot of fun, and to get to do it with the singlet on is icing on the cake. I appreciate all the local support out here today and runner-up is a good start to the season. Bring on Bells."
1 – Kelly Slater (USA) 18.56
2 – Joel Parkinson (AUS) 17.47
SF 1: Joel Parkinson (AUS) 18.17 def. Michel Bourez (PYF) 15.80
SF 2: Kelly Slater (USA) 19.37 def. Mick Fanning (AUS) 18.60
Revised ASP standings
1. Kelly Slater (USA) 10000 pts
2. Joel Parkinson (AUS) 8000 pts
3. Mick Fanning (AUS) 6500 pts
3. Michel Bourez (PYF) 6500 pts
5. Taj Burrow (AUS) 5200 pts
5. Julian Wilson (AUS) 5200 pts
5. Bede Durbidge (AUS) 5200 pts
5. Matt Wilkinson (AUS) 5200 pts