Berntsson began skating at age 5. Though he started out doing both hockey and figure skating, he soon focused exclusively on the latter. Berntsson won the Swedish junior national champion in 1996–1997, a title he would go on to win the next two season as well. He skated on the international junior circuit starting in 1998–1999. In the 1999–2000 season, still skating primarily as a junior, Berntsson won the national title and made his senior debut at the European Championships, finishing 21st. He finished in the same place for the next two seasons, and failed to qualify for the free skate at the 2001, 2002 and 2003 Worlds.