The entertainers had been friends for years before starring alongside each other in the 2019 Disney+ Christmas movie, Noelle.
"Anna has been dating Bill quietly for over a year," the insider told People on Thursday. "They met years ago. She's hosted Saturday Night Live and they've done a movie together, but they got together well after the movie."
The source said that the pair's relationship has not been difficult to keep under wraps for two main reasons.
"They are both very private people, and with the pandemic it was easy to keep it quiet," they explained.
Known for excelling in comedic roles, the insider went on to say that Kendrick and Hader are "both hysterical so they must keep each other laughing all of the time."
The SNL alum's last public relationship was with Rachel Bilson. He and the O.C. actress ended up parting ways amicably in July of 2020 after making their couple debut at the Golden Globes six months prior.
What's interesting is that the exes had actually co-starred in the 2013 romantic comedy, The To Do List, which was directed by Hader's then-wife, Maggie Carey.
Bill and Maggie were married from 2006 to 2018 and share three daughters: 12-year-old Hannah, 9-year-old Harper and 7-year-old Hayley.
At the time that Hader was ending things with Bilson, Kendrick was coming to some realizations of her own.
While discussing her HBO Max series, Love Life, Anna gushed that she was "just so happy that I'm clearer now about what I will accept from people in my life."
"The real arc that we wanted was to show how we learn from each relationship. Even though our relationships end, it doesn't mean they are complete failures," she said at the time. "People come into your life and you grow and they change you, and what you learn from them might be really positive or it might be negative."
As they say, timing is everything.