The YouTube star revealed her thoughts on the two pop stars' relationship, which coincided with promoting their duet "Senorita."
"They're super in love, they're actually so in love," the 21-year-old exclaimed while leaving a club in Los Angeles on Tuesday. "Off camera, they are, like, obsessed with each other. I've honestly never felt so single."
Although Mongeau never specified where or when she witnessed the sparks flying between Cabello and Mendes, she said they appeared so enamored by each other that she felt "like a 30th wheel." The couple acted "like they had no idea anyone else was in the room," according to the "FaceTime" songstress.
And Mongeau would be the one to fact check their coupling, as she is no stranger to having her romantic endeavors deemed suspicious.
"Honestly, being someone who has had a lot of people speculate if my relationship is real or fake, seeing them, like, really in love, was awesome," she admitted.
Last year, Mongeau announced she was engaged to fellow YouTube star Jake Paul after a very brief courtship. They wed a month later in a bizarre ceremony, which fans could stream for $50 to $75, according to TMZ.
Critics believed the whole thing was a clever ruse to remain relevant, especially when a soundbite from Mongeau's MTV YouTube show "Tana Turns 21" -- which debuted around the time of the wedding -- included her admitting their nuptials were "for fun and content."
Later, she admitted they were not legally married, but she "truly loves Jake."
Back when the news of their engagement was revealed, Bella Thorne posted a series of tearful photos of herself to Instagram, as she had reportedly broken up with Mongeau a few months prior. The former Disney star and Mongeau had dated for about a year, and were involved with rapper Mod Sun in a "throuple."