"I think it's been fascinating, and frightening, and really sad for our country," she said Friday on her show "Morning Joe."
"America's leaders and allies are asking themselves yet again whether this man is fit to be president. We have our doubts, but we are both certain that the man is not mentally equipped to continue watching our show, 'Morning Joe,'" they wrote, arguing Trump has an "unhealthy obsession" with the show and suggesting he stick with watching "Fox & Friends," the morning show on rival network Fox News.
As "Morning Joe" was wrapping up its Thursday show, Trump tweeted about "Crazy Mika" and "Psycho Joe" and made comments about Brzezinski's appearance.
Trump regularly slammed the media as "fake news" during his presidential campaign, but had a friendlier relationship with the "Morning Joe" hosts throughout his candidacy, often dialing in to the show for early morning interviews.
"You guys have been supporters, and I really appreciate it," Trump said to Scarborough and Brzezinski in February 2016. "And not necessarily supporters, but at least believers."
But as Election Day neared, the candidate's relationship with the hosts soured, with Trump at one point calling them "two clowns."
Trump has a history of publicly humiliating women and making sexist remarks. In October 2016, The Washington Post published a recording of Trump bragging about inappropriately grabbing women and trying to have sex with a married woman.
The White House defended Trump's latest remarks, with spokeswoman Sarah Huckabee Sanders saying the president "fights fire with fire" and arguing people shouldn't be surprised by his tweets.
On the show Friday, Brzezinski insisted she is "fine," saying, "My family brought me up really tough."
"This is absolutely nothing ... for me personally," she said.
Trump reacted to "Morning Joe" again on Friday in a tweet:
This article has been updated with Trump's Friday tweet.