A camera crew captured the humpback whales breach right alongside Mitchell's surfboard.
"I saw them come up and submerge really quickly, and I thought that might be the end of it, all of a sudden they popped up right near me," Mitchell said.
"I actually sort of got scared, paddled away, tried to get away from them because they're such big creatures," he said.
A film crew was following mitchell as part of a documentary called "Living in Liquid."
Mitchell is in Redondo Beach this week as he prepares for an upcoming race. He said his encounter with the whales was one he'd never forget.
"I definitely feel special," he said.
"Even without the cameras, in my memory, it would have been, memory for life, but especially to have it captured on film is even more special."