An electropop ballad, the song's lyrics address Finneas losing touch with a childhood friend and regretting doing so, as well as wondering why he didn't do anything about it.

In an interview with WBUR-FM, Finneas explained the song is an autobiographical about a friendship he had that lasted all through high school and had fallen apart.

[2] "I Lost a Friend" was released for digital download and streaming as the second single from Finneas' debut EP Blood Harmony through his record label OYOY on May 3, 2019.

[8] "I Lost a Friend" features minimalist instrumentation that consists of bass guitar, piano, drums, layered harmonies, an aggressive beat, and synthesizer.

[9][10][11] Finneas later explained the production in an interview with Pitchfork, saying: "The coolest thing I felt that I found was I had this flute patch that I arpeggiated.

He discussed the pre-chorus further, saying how common it is for him and others to stay awake at night "replaying fights", and also said, "I find that when I'm in an argument and I'm angry, I can't even form a sentence well.

[10] Brittany Spanos of Rolling Stone said the track "carries a universal appeal with many fans of the song messaging him about their own friend break-ups in recent months".

[11] For NME, Will Richards was less positive towards the song, saying it "starts as a run-of-the-mill, earnest singer-songwriter tune, but soars towards the end of stabs of distorted bass," comparing "I Lost a Friend" to Billie Eilish's debut studio album When We All Fall Asleep, Where Do We Go?

[16] In the visual, Finneas wanders drunkenly through a field as the sunset lowers and darkness approaches, wearing a suit and tie.

[15][16] While Finneas continues to across the field, human forms rise and stumble to follow him as he tries to understand what he has done wrong, singing, "How'd the hell I lose a friend I never had?

[11] That same month, Finneas performed "I Lost a Friend" at the Austin City Limits Music Festival and was backed up by a live band.