A teary-eyed McCarthy, 41, broke the news about the engagement on her morning talk show The View Wednesday.
"This weekend I was sitting with Donnie ... and he went into the other room and out came Evan [her 11-year-old son] with a card that said 'Will' and he handed it to me," she said. "Then Evan ran away and Evan came in with a card that said 'You.' ... I just started crying and then Evan came back into the room with the word 'Marry' -- but spelled wrong."
"The last time, Donnie came out with Evan and Donnie's shirt said 'Me' ... and he got down on his knee," she continued.
McCarthy "of course" said yes.
"In that moment Evan yelled, 'I have another dad!' and it made all of us cry," she said crying.
Wahlberg came out from backstage right after the former Playboy bunny finished telling the story and hugged her.
The couple has not yet set a wedding date. McCarthy and the Blue Bloods star have been dating since July 2013.
You can check out a video of McCarthy sharing the news here.