[8][9] The song contains nostalgic lyrical references to the titular year along with a snare-heavy beat, "fuzzy sawtooth bass and sparking Eurodance keys".

[10] Sasha Geffen of Pitchfork stated that the track is "more concerned with the act of remembering than with the specifics of the year it calls up itself" and "fits alongside the rest of Charli XCX's sleek, forward-looking pop songs".

[10] The song also references Britney Spears' 1998 debut single, "...Baby One More Time", as suggested by Max Martin.

[15] Charli XCX and Sivan tweeted lyrics from "1999" addressed to each other on Twitter before sharing the cover art and title of the song.

[16][17] The music video was released on 11 October 2018, and features Sivan and Charli XCX in various homages to 1990s pop culture, including the iMac G3 and Steve Jobs, Spice Girls, Britney Spears and Justin Timberlake, New Radicals, Eminem, the Nokia 3310, Rose McGowan and Marilyn Manson, the films Titanic, The Matrix, American Beauty, and The Blair Witch Project, the Dancing baby, the video game The Sims, and the music videos for the songs "Say You'll Be There", "Waterfalls", "Thinking of You (I Drive Myself Crazy)", and "I Want It That Way".