Jemma Griffiths was born in Penarth, Wales, a small town near the Welsh capital, Cardiff on 18 May 1975. Jem originally found her passion for singing and songwriting whilst attending Stanwell School. During her early school years she explored her musical interests with the family piano, penning her first song at the age of thirteen. Jem has three siblings, older sister Chloe, (whose photograph appeared on Jem's first extended play It All Starts Here...), younger sister Georgia, who provided vocals for the Welsh indie/punk band Weapons of Mass Belief alongside brother Justin, who also goes by the name Yestyn, of Glass Pear.
In 1993, Jem moved to Brighton to study Law at the University of Sussex. This decision was due in part, because her father was a lawyer, and also because her older sister had attended university before her. Jem assumed it was the best route for her to take at the time, however, she had no intentions of becoming a lawyer, and continued to maintain her passion for music throughout the course of her education. Both during her studies and after graduating in 1996, Jem spent time as a club and festival promoter and also began working as a DJ in Brighton's lively club scene. With regards to her university education Jem stated that she "went to class as little as could without being kicked out".
In early 1998, alongside Adam Freeland, she co-founded and helped run the specialist breakbeat/hip hop record label Marine Parade. However, after spending two years acting as agent and promoter for other aspiring musicians, she soon felt that she had neglected her own musical career for too long. Therefore, in November 1999 she surprised her colleagues by leaving Brighton and returning to Wales, where she assembled a mobile recording studio and focused on developing her own individual songwriting and music production skills, completing a collection of four demos that acted as the basis of her break into the music industry.
In 2000, Jem moved to London where she met Sugababes manager Sarah Stennett, with whom she began a professional relationship. Stennett introduced her to other writers and producers. Two days into her first writing session with acclaimed electronic producer Guy Sigsworth (Björk, Frou Frou), the song "Nothing Fails" was born, with the lyrics eventually being picked up by Madonna for her 2003 studio album, American Life. In 2001, Jem travelled to Brooklyn, New York City, teaming up with hip hop producer Ge-Ology (Mos Def, Talib Kweli) and co-producer Yoad Nevo, where they succeeded in fusing her many musical influences to create a fresh and distinctive new sound.
In March 2002, during a trip to Los Angeles to visit friends, Jem stopped by independent radio station KCRW to leave a demo of her song "Finally Woken" in Nic Harcourt's mailbox. Credited with being the first American DJ to play Coldplay, Norah Jones and Dido, Harcourt immediately began playing it on his show Morning Becomes Eclectic, whose popular listeners included big names in the music business. The track quickly became one of the most listener-requested songs at the station, landing Jem in their top 5 played artists of 2003, all prior to any record label being attached to her name.