"I'm sad. I wish we had at least another year for the writers to have that much time to ramp up to the ending," she admitted. "We sort of found out sooner than we thought we would hear. We thought, 'Surely they're going to want more Mom.' And they decided not."
Janney added her take on why the sitcom was not renewed past season 8.
"There are so many reasons behind that, most of them probably money," she shared. "But I know that [creator] Chuck Lorre is writing the final episode, and we're now filming the third to last episode and, you know, I'm having a lot of moments where I'm just standing on set, taking it in and looking at all the faces I've looked at for eight years."
Reflecting on the huge chunk of time she's spent with the cast and crew of the show, Janney said, "No matter how much I prepare myself for it, I know I'm just going to be tears, buckets of tears. Because it's just been so incredible."
Mom's final episode will air May 6 on CBS. When it was first announced last month that the comedy would be ending, Janney took to social media to reflect on the series.
"Getting to be part of MOM these past eight years... showing the laughter, love and hope that can come with recovery, has been one of the great honors of my life," she wrote at the time.
The sitcom's cancellation comes six months after Janney's co-star, Anna Faris, left the show after seven seasons "to pursue new opportunities," leaving Janney -- who has won two Emmys for her performance -- as the sole lead for season 8.