The outspoken host didn't shy away from sharing his thoughts with a number of different acts on Tuesday.
While Mandel has been known to share controversial feedback, his critique for Mzani Youth Choir began with a great deal of praise, as the comic told the singers that he loved them and their work and thought they are among his "favorite choirs ever in the history of AGT."
However, Mandel went on to tell the singers, "This wasn’t my favorite number you've done out of the three numbers [on the show.]"
Mandel's feedback was quickly drowned out by booing -- both from those in the audience and by fellow judge Sofia Vergara.
"Oh what a wonderful cheerleader you are, Sofia," Mandel shot back, before telling the choir that, just because he didn't care for the song, "That doesn’t mean that you’re not worthy of being here in the finals and being voted for."
The Mzansi Youth Choir was one of five Golden Buzzer acts to make it to the finals this season -- alongside dance group Chibi Unity, who also got some strong criticism from Mandel when he said the performance was "at best mediocre."
This elicited another massive round of booing and hissing from the audience, and a stern glare from both Vergara and Heidi Klum.
"I'm sorry, but I didn't find it amazing," Mandel shot back.
ET's Denny Directo spoke with Mandel after the show, and while he didn't mention the criticism or the booing, he did share some love for the acts in general and particularly for his own Golden Buzzer act, Murmuration, which performed last in the line-up.
"The thing is that we had, in this finals, we had five golden buzzers competing. We've never had that in the history of a final," Mandel shared. "So the level of competition was bigger and better than it's ever been."
"Mine closed the show, and what a closing it was! It was probably one of the most memorable performances in my fourteen years on AGT," he added. "I loved it. I loved the message. I love the visuals, I love the sound. I loved everything about it."
"I think that this is the beginning of something really special," Mandel added. "Murmuration can take technology, can take music, take lightening, can take whatever and add [to it]. You're seeing just the beginning and the seed of an idea that is just going to explode."
Meanwhile, Vergara spoke with ET as well and reflected on how much she enjoyed being a part of the show this year.
"I mean, it's amazing. The whole, the whole season, to have this job that we have, that we're like watching these people, these amazing talents... it's the most entertaining job that I've ever had," she shared. "I'm very lucky. And it's also a pleasure to see how these people, through the season, they become even more amazing."
America's Got Talent crowns a new Season 18 champion on Wednesday, with a live finale kicking off at 8 p.m. ET/PT on NBC.