It's the sixth time Howard has won the event.
Howard broke a 2-2 tie by scoring four points in the fifth end.
Martin cut the margin to two in the sixth end, but Howard put up one more point in the seventh to seal the victory.
The win leaves Howard and his teammates Wayne Middaugh, Brent Laing and Craig Savill as the only rink still in the running to capture the $1 million prize available for sweeping all four Grand Slam events.
"We're off to a great start, and to win a Grand Slam is the pinnacle," said Howard. "You have the best of the best. All the best teams are here and to win that, you have to be doing something special."
Meanwhile, Ottawa's Rachel Homan beat Scotland's Eve Muirhead 7-5 in the women's final.
Homan led 5-2 after four ends, but Muirhead made a late rally with two in the seventh end to cut the deficit to 6-5 before Homan held on with a point in the eighth.
"We had a couple of losses early, but we were learning the ice and learning from our mistakes," said Homan. "We just kept building, got a couple of breaks and took it one game at a time."
Just making it to the final was a feat for both rinks. Homan and Muirhead each finished the round robin 2-2 and needed to win tie-breaker matches before they could advance.