"In My Blood" is a rock and pop rock ballad, whose acoustic opening was described as "soft and despairing". As the song progresses, "an insistent drum comes in" before Mendes' voice rises on the chorus. A Rolling Stone contributor noted that the chorus features "shuddering drums" and "throbbing electric guitars", with "choral backing vocals" that "add soothing textures to the final hook". Mendes's vocals spare from A2 to C5 in this song.
