Taking to the stage in a beige jacket, blue shirt and yellow slacks -- giving off serious Mick Jagger vibes -- Styles compared hosting SNL to dating and being in a relationship.
"And just like all my serious relationships, we're gonna spend one incredible night together and then never see each other again," Styles said with a charming smile.
To get into his comfort zone, Styles sat down at a black grand piano and delivered the rest of his monologue while "playing" a light jazz tune.
"I'm not in a boy band anymore. I'm in a man band," Styles joked, reminding everyone how he used to be in One Direction.
"How crazy would it be if they were here tonight?" Styles said, to the cheering of the audience. "They're not though. But wouldn't it be crazy if they were? But they're not."
He went on to say that he still thinks of his former band mates as his "family."
"I love those guys. Niall, Liam, Louis..." Styles said, struggling to come up with the name of the former One Direction member Zayn Malik, who left the group long before they went their separate ways. "Uh... Ringo! Yeah, that's it."
"I'm so grateful Simon Cowell grew us in those test tubes," Styles added.
It was a great start to what quickly went on to become one of the season's best episodes, largely because of Styles' enthusiasm.
After crushing his hosting duties, an eyewitness tells ET that Styles showed up to the show's after-party wearing the same outfit he delivered his monologue in. Late Late Show host James Corden was spotted chatting with Styles in one of the corner booths overlooking 30 Rock. The two also stopped by Pete Davidson’s table to chat with him and his rumored girlfriend, Kaia Gerber.
Other celebs at the party included Jon Hamm, Tiffany Haddish, Zoey Deutch and SNL cast members Kate McKinnon, Beck Bennett, Aidy Bryant, Mikey Day, and more.
SNL airs live coast-to-coast Saturdays at 11:30 pm. ET, 8:30 p.m. PT on NBC.