Born in Yokohama, Japan, Masaru Shiina was taught how to play the electronic organ from his parents when he was growing up. He was at one time a member of a Megadeth cover band, who also played J-pop music. Shiina enrolled at Dokkyo Junior High School. After being rejected by 46 companies, including a jewelry company, a finance company, and a fast food company, he was hired by Namco (now Namco Bandai Games); he had little prior experience with video games.
Shiina mostly composes jazz, orchestral, fusion, and rock pieces. He has said that he can write music in any genre, as long as he is motivated. He also stated that "when I write a song, the time it takes me depends on whether I know the singer's natural voice. The genre doesn't matter…" Shiina feels that there are two types of composition: "some tracks can stand alone as musical pieces, and some only work when you hear them in the game." When composing vocal songs and instrumental tracks, he sings them out loud. For the soundtrack to God Eater, Shiina stated that the "creative nature of the story pushed me to be creative with the music as well. I looked around for a variety of instruments from many countries." Some of the styles and instruments featured in the score include Japanese traditional music, African rhythms, and the Indian sitar.