The British singer and "Stranger Things" star were first linked back in August as they attended a production of "The Lehman Trilogy" in London together, and confirmed their relationship with a passionate PDA session while on a stroll around New York earlier this month.
Many fans were left stunned by the somewhat unusual pairing, and David's pal, "The Avengers" star Sebastian, has admitted he was one of them - and was so perplexed by the reports that he ended up Googling it.
Asked during a chat with America's Life & Style magazine what his last Internet search was, Sebastian replied: "I googled David Harbour. Well, someone told me he's seeing Lily Allen and I was like, 'Lily Allen! I haven't seen or heard of her for a minute.' So, on my way here I actually googled that."
"I wanted to make sure I got his girlfriend right, because you know, I haven't seen him in a while so I don't know what he's been up to."
Lily and David's romance began shortly after his split from "Fantastic Beasts and Where to Find Them" star Alison Sudol. Lily had been single since her three-year relationship with rapper Meridian Dan came to an end in December 2018.