The pop star kicked things off by discussing the EPA's Scott Pruitt spending over $1,500 on 12 customized fountain pens.
"I mean, in his defense, Scott Pruitt thought EPA stood for Expensive Pen Agency," Mendes joked. "$1,500 worth of pens! That's still less than I've already stolen from the 'Late Late Show' office supply cabinet."
"Why would you waste $1,500 on pens? Honestly, like, you could use that money to buy 3,000 tacos from Jack in the Box," he added. Then an angry James Corden came storming out from behind the curtain.
"Whoa, whoa, whoa, whoa," Corden exclaimed. "Shawn! Shawn! What is going on here?"
"I'm here all week, so I thought I could kick things off by saying some topical jokes like you do," Mendes replied.
"No, no, Shawn, no," Corden said. "I am thrilled that you are here all week, OK? But the monologue is mine. You're gonna do a song every night, you helped me get to work yesterday in 'Carpool,' you can do sketches, but this -- right here -- this is James time. You can do whatever you want, but I do the monologue, OK?"
The singer replied, "I was thinking we could do a Shawnologue," which was met with screams from the audience. "It's like a monologue, but I do it."
Corden was displeased and demanded Mendes exit the stage, but before the latter left, he altered the "Late Late Show" sign behind him to read "The Late Late Shawn."
Snap, Crackle, Pop -- Shawn's Voice!
In an epic surprise to the man who stole his show, Corden compiled a video montage of all the times Mendes' voice cracked while he was performing on stage.
"I did not approve this," Mendes said laughing before Corden played the video. Mendes was adamant that the last clip was "fake," but Corden wasn't convinced.
Shawn's Album Inspo and What Corden Thinks He Should've Named It
Mendes just released his third studio album -- "Shawn Mendes" -- and it's already at the top of the charts. Mendes explained that his self-titled record was inspired by a multitude of genres but said it wasn't done consciously.
"I would wake up at the gym, listen to rap, then listen to [John] Mayer on the way home from the gym, and then listen to Top 40 radio later that night," Mendes explained. "And I realized that night that the genres were no longer there for me as a listener. And then I would get into the studio, and one day I'd wanna make a rock song, and the next day I'd wanna make R&B, and I realized that if you truly love what you're making and the conviction in what you're doing is there, then it'll all sound cohesive."
Despite loving the record, Corden admitted he was a little hurt that Mendes didn't call him up to ask for suggestions prior to naming his album. Among the list of alternative album names were "Boys to Mendes," "Be-shawn-cé," "LeBron Mendes" and "It's Raining Mendes."
"He's getting worse as the night goes on," Mendes said as he looked toward the audience.