"I'll Be Lovin' U Long Time" is a "mid-tempo", "party jam", that draws influence from pop, R&B and soul music genres. Critics elaborated on the song's "soul" influence, with Melissa Ruggieri from The News & Advance described it as a "soul-thumper", while Digital Spys Nick Levine called it a "nod towards classic soul". The song's title is derived from the popular line, "me love you long time", from the 1987 war film, Full Metal Jacket. Additionally, its hook and instrumentation is derived from sampling DeBarge's "Stay with Me". Critics also noted how "I'll Be Lovin' U Long Time" "recalls" the melody riff and chord progression of the Hill Street Bluess theme song. The song's main source of production comes from its "lush keyboard work", and Carey's usage of the "double voice", which she uses throughout the song's bridge. Carey described the effect as "layering her voice", so that the bridge would sound like a "swooning bank of a hundred Mariahs". The song was written by Carey, Aldrin Davis, Crystal Johnson and Clifford Harris, and features Mark DeBarge and Etterlene Jordan on the songwriting credits, due to the inclusion of the samples. "I'll Be Lovin' U Long Time" was produced by Carey and Davis (DJ Toomp), and is arranged in the key of D-flat major, with Carey's vocal range spanning from the low-note of A3 to the high-note of D5.
