On the song, The Chainsmokers said "It's about the hardships of relationships but never giving up on your ride or die."
The song is written in the key of C major with a common time tempo of 90 beats per minute with a chord progression of C – Fsus2 – Am. The vocals span from G3 to A4 in the song. The lyrics were written by Taggart and Swedish songwriting duo SHY—made up of Sara Hjellström and Nirob Islam (aka SHY Martin and SHY Nodi, respectively)—while the production was handled by The Chainsmokers.
Isis Briones of Teen Vogue stated "When you look beyond the catchy beats, the song explores a situation arguably worse than wanting to get back together with your ex: The idea of a fading relationship. The DJs open up a door of emotions many can relate to. As tragic as the initial lines may be, the rest of the song pulls through with the most heartfelt happy ever after you'll hear."Idolator's Mike Wass said "[All We Know] picks up where 'Closer' left off" and went on to say "it's almost as if The Chainsmokers took 'Closer' and refined it. That might be a little safe, but it's also very smart." Deepa Lakshmin of MTV labeled it a "bittersweet tune about a relationship that's not quite going anywhere" and said it is "basically a sequel to 'Closer'."
The music video for the song directed by Rory Kramer was released to YouTube on November 17, 2016. The music video included cameo of The Chainsmokers as customers and Phoebe Ryan as a cashier. The video begins with a man (portrayed by Casey Deidrick) fleeing out from his girlfriend's house after receiving a desperate phone call. He buys a bottle of liquor in a store and then smashes the bottle, his phone, and a trash can cover. Feeling drawn out, he hitchhikes a car as the sun was about to rise. The video ends with his footage of walking on a mountain trail as cutbacks of his prior events last night were played in rewind, culminating in meeting his girlfriend back at the mountain peak.
The video's shooting method is found footage, in which a camera is strapped to Casey's waist to record the music video.