On Saturday, Honnold became the first person to free-solo climb the mountain known as El Capitan in Yosemite National Park.
"Free solo" is climber-speak for no ropes, no safety gear.
"I've never dreamt of anything bigger," Honnold said.
Honnold, 31, made the historic climb under the eye of a National Geographic film crew, which shot footage of his ascent up El Capitan's nearly 3,000-foot granite wall for an upcoming documentary about him.
Iconic for its near vertical cliffs, Yosemite's El Capitan was believed to be impossible to climb until 1957, when American rock climbing pioneer Warren Harding made it to the top with two aides. But they weren't using just their hands.
Honnold started his ascent on the mountain at 5:30 in the morning. A few hours later, he was at the top.
"It took a little under four hours. I was going quickly. I was very excited. I've been dreaming about this since 2009," Honnold said.
National Geographic, which dubbed the climb "the moon landing of free-soloing," said Honnold trained for more than a year in locations in the United States, Europe, China and Morocco. The climber also knows El Capitan very well.
"I've probably climbed the face 50 or 60 times," Honnold said. "I already held the speed record on El Capitan."
Honnold attempted to free-solo El Capitan in November, but abandoned the effort after about an hour, citing unfit conditions, according to National Geographic.
Honnold said he kept his plans low-key, with only a small group of friends and climbers privy to his plans to make history.
"We had no idea that he was going to do this," said Michael Honnold, Alex's uncle.
"He called and talked to his aunt a week ago, and he was actually 'on belay' when he was talking to his aunt. Said I've got to go," Honnold's uncle said. "On belay" means he was dangling from a rope while they were talking.
Honnold has been a wunderkind in the climbing world since 2008, after two game-changing rope-free ascents at Yosemite's Half Dome and Zion National Park's Moonlight Buttress in Utah, according to National Geographic.