According to Guinness, the epic feat eclipsed by more than 1 foot the previous record, set in 2008 by big-wave surfing great Mike Parsons at Cortes Bank in California.
NBC San Diego reported that McNamara was still shocked at the wave's size when he looks at video footage.
"This wave, if you fell, I don't think I would've come home," he said. "It's so dangerous, so much water moving in two different directions and it's all rocks. All the water is moving straight into the rocks."
McNamara, who has surfed professionally since age 17, said he had not planned on going to the beach at all on the day he set the record after spending the entire previous day in waves that "believe it or not were a little bigger."
He said friends woke him up early and persuaded him to come with them and that he went reluctantly -- feeling "beat up" from the day before and only planning to drive the jet-ski used to tow big-wave surfers out far enough to catch the break.
"They talked me into getting on my board," McNamara said. And when I got on the board and they pulled me up with tow rope everything just felt right, it was like magic."
McNamara said he was caught up in the moment as he rode the wave, then worked to stay in control of his board as the colossal wall of water drove him straight toward rocks where "if you fall, there's not much chance of survival."
The surfer said he knew the wave was big but was not sure he had set a record until it was made official last week at the Billabong XXL Global Big Wave Awards in Anaheim, California.
"I really just surf for the love of it and I wasn't looking to get the record. I wasn't looking to get any awards," he said.
"One the main reasons I'm doing this is to hopefully inspire at least one person to do what they love, to do what they are passionate about," McNamara said. "I am living proof you can do what you love as a career."
More content from msnbc.com and NBC News:
- Outrage, calls for action over anti-Muslim military course
- Florida woman gets 20 years over warning shot
- Video: 31 years later, a confession to murder
- Cities: Occupy protests cost taxpayers millions
- 'Tripawed' the 3-legged grizzly returns to Denali
- Houston reporter fired for stripping files discrimination complaint
- Son defies odds to walk into returning soldier's arms