Allen, 34, and Harbour, 44, were recently spotted in New York together sharing a kiss and a romantic stroll, which Just Jared shared on their Instagram account.
The pair kept it casual with their outfits. Lily wore a gray jumper paired with baggy jeans and sneakers. She wrapped her bag around her body and tied her short brunette hair up.
David wore a navy jacket with gray joggers and sneakers. They both had sunglasses on and appeared to enjoy their day around the city.
The "Hard Out Here" songstress also watched her new man host Saturday Night Live over the weekend and shared a photo of his arms to her Instagram story. She wrote "mine" on the image, subtly confirming their relationship, per Capital FM.
Since the news broke, social media reacted to them becoming an item.
"Lily Allen dating hopper from stranger things is the most random thing ever," one Twitter user wrote.
"I was today years old when I found out Lily Allen and David Harbour are dating & I'll be today years old when I die. This way, I can die the happiest I'll ever be because THIS IS PERFECT," another shared.
"David Harbour and Lily Allen are together??? I do not stan celeb couples but oh my god two of my favorite people," a third mentioned.
"David Harbour and Lily Allen have to be the most unexpected couple wtf???" a fourth account remarked.Earlier this month, Allen dropped a new single with DJ Spoony, "Sweet Like Chocolate." The track is a cover of Shanks & Bigfoot's popular single. It will be a part of Spoony's upcoming album, Garage Classical, which consists of British artists covering classic garage songs with an orchestra. The Inquisitr reported the fans' reaction to the release.
The project, which drops on October 18, includes other collaborations with the likes of Paloma Faith, Sugababes, Gabrielle, and Emeli Sande, to name a few.
In the U.K., Lily has achieved three No. 1 singles — "Smile," "The Fear," and "Somewhere Only We Know" — and a total of 10 top 10 singles.
All four of her studio albums have entered the top 10 in her home country. Her second and third releases, It's Not Me, It's You and Sheezus, both topped the charts.
Last year, she became an author and released her memoir, My Thoughts Exactly. The book was a success and became a Sunday Times bestseller.To stay up to date with Lily, follow her Instagram account.