It wasn't easy. The US climber had powered on through the dark and dealt with a deep cut to her head to become the first woman to free climb the iconic rock face's notoriously difficult Golden Gate route in under 24 hours.
The 34-year-old completed the mammoth task in 21 hours, 13 minutes and 51 seconds, and in doing so also became just the fourth woman to free climb the 3,200-foot monolith in Yosemite National Park within a day.
"I'd just fantasized about it and imagined what it would be like and it was pretty much exactly what I envisioned," an ebullient Harrington told CNN Sport.
"It was incredibly quiet. It was super dark. All the stars were out. It was just this really serene, peaceful experience.
"I keep telling people that when great sporting achievements happen, a lot of times there's an audience or there's a stadium.
"With climbing, it's not so much like that. It was just this really quiet special moment in this magical place. And it's something I'll never forget."
Free climbers use just their hands and feet to climb, with a rope to catch them if they fall.
Such a high-stakes, and potentially life-threatening, challenge requires years of preparation, both mentally and physically.
Harrington had climbed this particular route over the course of six days in 2015 and had subsequently tried three times to complete it within 24 hours.
However, an attempt last year ended in disaster after Harrington fell 50 feet, hit her head on a ledge, and suffered concussion.
"It was very scary. It was very serious initially and it turned out that I got really lucky and I did not suffer any long-term injuries," she added.
"It's definitely a mental struggle, coming over that hurdle, coming back into this year and trying again."
Ever since she started climbing as a 10-year-old, El Capitan has enchanted Harrington. She's spent numerous hours practicing different pitches (various sections of a climbing wall) to become accustom to arguably the most famous rock in the world of climbing.
In recent years, El Capitan has captured the public's imagination after Alex Honnold climbed it without a safety rope in the Oscar-winning documentary Free Solo, which was released in 2018.
Honnold was one of two people to help Harrington on her successful ascent, the other being her fiancee Adrian Ballinger -- an experienced climber and mountaineer.
Both climbers took turns assisting Harrington, with Honnold helping for the first two thirds of the climb.
The pair essentially "caterpillared" the wall, with Harrington attached to the top of the rope and Honnold to the bottom. The technique helps speed up the ascent, but is risky.
Ballinger then took over belaying duties on the harder, steeper sections near the top where Harrington knew she would have to slow down.
The couple then took it pitch by pitch, with Harrington waiting for her partner to catch up before taking on another section.
"He's been the one who supported me while I practiced on the wall," said Harrington of Ballinger.
"He's my partner in life as well and I just knew that it was going to get harder and I knew that it was going to get emotionally difficult. I just needed by my person there for me for that."
'Blood pouring down my face'
If the challenge itself wasn't enough, Harrington was pushed to her limits when she fell 12 hours into the climb, her hands slipping when at approximately 2,800 feet off the ground.
"It felt like it was going to be a normal fall, a safe fall, the rope was going to catch me. Everything was going to be fine," she said, admitting her mind might have been focused on the upcoming pitch.
"The next thing I know, I just saw black and I felt the wetness of blood pouring down my face.
"I realized that I had hit my head again and that was definitely a rock bottom moment."
The gash on her head was deep and it seemed as though the challenge would have to be abandoned yet again.
But, this time, she hadn't suffered concussion. A makeshift plaster stemmed the bloodflow and, determined not to let another fall steal her dream, Harrington gave the pitch one more go.
"I just had one of those crazy, out of body, flow state experiences, where you don't really even think and you're just sort of climbing and almost watching myself climb from a distance," she said.
"It was a pretty magical feeling, actually.
"To have that really dramatic thing happen and get injured and have all that deja vu from the previous year and then be able to turn it around and climb perfectly and then continue to the top."
Harrington is able to laugh that the fall helped write the "perfect script" to her climb, which was inspired by pioneering rock climber Lynn Hill, the first person to free climb El Capitan in under 24 hours, back in 1994.
The climb also served as the perfect distraction from what was arguably one of the most important days in US history.
As Harrington scrambled up the rock face, millions of Americans waited to see who their new president would be.
Harrington says she had always planned to be ready in early November and admits attempting the feat on election night was semi deliberate.
"Everyone in this country was feeling a lot of uncertainty and a lot of lack of control," she said.
"We had no control over what was going to happen and everyone was just sitting there doom-scrolling on their phones.
"I think, in a way, I was like, 'This will be a good distraction, it'll be a good way to not focus on that and not drive myself crazy.'
"It worked out, I definitely thought about it a lot less than most people."
Where it all began
As an only child, Harrington's competitive edge began with a healthy rivalry with her cousins.
She remembers the first time she competed against them on a small, local climbing wall.
"The instant I stepped on the wall, there was this feeling of 'Oh, this is what I want to do,'" she said.
"I belonged there in a way and I just remember telling my dad that I wanted to climb."
She initially excelled at indoor competitions, becoming the US sport climbing champion five times and twice winning the North American championship.
The transition into the world of outdoor climbing only began after signing with the North Face's athletic program, and her climbing future was sealed when she met her partner while climbing Mount Everest in 2012.
She subsequently moved to California and began climbing in Yosemite National Park.
"People talk about conquering your fears, they talk about beating your fears, making your fears go away, I don't ever really feel that. I feel fear quite frequently, honestly," she said.
"I work through it by just accepting it and trying to understand why it's there and then taking steps to move forward.
"We should be less afraid to be afraid. It's a very valid emotion and it's something we shouldn't shy away from. In a lot of ways, we can use it as fuel and as strength."
Harrington is currently staying put amid the pandemic but hopes to be back climbing around the world when it's safe to do so.
There are a number of challenges in Europe that she's keen to take on, although not all will be as dangerous as her last.
Harrington says around 90% of her climbing is safe (relatively speaking) and says more dangerous trips come after a lot of thought.
That being said, she hasn't ruled out a return to Yosemite.
"There's plenty of other routes on El Capitan to challenge myself with," she said, smiling.