"This Is It" is a pop ballad with a simple mid-tempo keyboard that lasts three minutes and 36 seconds. The song begins with Jackson singing, “This is it, here I stand / I’m the light of the world, I feel grand / Got this love I can feel / And I know yes for sure it is real” and then he tenderly hits his signature high notes about a new lover he feels he's known "since 1,000 years." Jackson's brothers Jackie, Tito, Jermaine, Marlon and Randy (credited as The Jacksons) are featured as background vocals throughout the song. Background music includes fingersnaps, a "soaring chorus," big-build strings and "bubbly guitars." The song is played in the key of Bb major. Throughout the song, Jackson's vocal range spans from F3 to C5. The song's tempo is moderate, and its metronome speed is 96 beats per minute. According to the music published on Musicnotes.com by Alfred Music Publishing and the Winnipeg Sun, the song is written in common time. The lyrics "toggle between grandiosity" ("This is it, here I stand / I'm the light of the world, I feel grand") and "shy, romantic yearning" ("I never heard a single word about you / Falling in love wasn't my plan"). Darrly Sterdan of the Winnipeg Sun felt that Jackson's voice and the background music sounded "like a demo that's been fleshed out with overdubs and doowoppy backup vocals from his brothers." Anthony McCartney, of the Associated Press felt that the finger-snap percussion "echoes" that present in Jackson's 1982 song "The Girl Is Mine" with Paul McCartney.
