The former One Direction frontman pulled double duty as both the TV show's host and musical guest.
During his monologue, the vocalist named all of his former bandmates except for Zayn, who he jokingly referred to as "Ringo."
For his musical performance, Styles debuted the latest cut from his much-anticipated sophomore album, entitled "Watermelon Sugar."
"Tastes like strawberries on a summer evenin'/And it sounds just like a song/I want more berries and that summer feelin'/It's so wonderful and warm," he crooned over a horn-filled melody.
Earlier on "Saturday Night Live," the British musician also performed the previously released ballad, "Lights Up," which will also appear on his upcoming new album.
"Fine Line" will arrive on December 13, more than two years after his self-titled debut album.
Although details about the full-length are still scarce, Styles opened up in a recent Rolling Stone cover story about the genesis process of "Fine Line."
The album, which he described as "all about having sex and feeling sad," was notably recorded over a six-week period in Rick Rubin's Shangri-La studios in Malibu, California.
For his sophomore studio effort, the 25-year-old vocalist collaborated with producers Tyler Johnson and Jeff Bhasker, as well as with writers and musicians from his first LP such as guitarist Mitch Rowland, drummer Sarah Jones and producer Kid Harpoon.
In support of "Fine Line," Styles will embark on a massive world tour next year, in which he will be joined on stage by Jenny Lewis, Koffee and King Princess.
The North American leg of the trek will kick off on June 26 in Philadelphia, Pennsylvania, with a string of dates also planned in the UK and Europe.
In the meantime, discover Styles' live rendition of "Watermelon Sugar" from "Saturday Night Live."