John Powell was born on 18 September 1963 in London. As a child, he played the violin and viola. His skill in the violin allowed him to study at Trinity College of Music, central London (now the Trinity Laban Conservatoire of Music and Dance in Greenwich, London). There, he met John Ashton Thomas, who orchestrated many of Powell's scores. Powell played for "Faboulistics", an amateur rock and roll band.
After finishing college, he composed music for commercials, which led to a job as an assistant to the composer Patrick Doyle on several film productions, including Much Ado About Nothing. In 1995, Powell co-founded the London-based commercial music house Independently Thinking Music with Gavin Greenaway, which produced scores for more than 100 British and French commercials and independent films.