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.

His results continued to improve the next season, when he moved into the top ten at Europeans for the first time in his career and was 14th at Worlds.

In the 2005–2006 season, Berntsson finished 11th at both his Grand Prix events, and lost the national title to Adrian Schultheiss.

At the 2007 Europeans, Berntsson finished 10th, and he was a career-best 9th at 2007 Worlds, where he received a standing ovation following his disco-themed long program.

[6] He finished 8th at the 2009 Europeans but 20th at Worlds, which, combined with Adrian Schultheiss's 18th place, meant that Sweden would have only one men's entry at the Olympics.