Berger joined the national biathlon team in 2001. During the 2004 Biathlon World Championships in Oberhof, Germany, he won silver medals in the 15 km mass start and the 4 × 7.5 km relay. Berger also won two gold medals in the military world championships that same year (cross-country and patrol). Berger finished fifth in the 2004 overall World Cup, and won several gold medals from the Norwegian Biathlon Championships. At the 2007 Biathlon World Championships in Rasen-Antholz, Italy, Berger was part of the Norwegian team that won silver in the 4 × 7.5 km relay. After several disappointing races, mainly due to unstable shooting, Berger was thrown off the Norwegian national team ahead of the 2008/2009 season. He revenged this by retrieving two world cup victories in sprint events, a silver in the World Championship sprint, and contributed to the gold in the men's relay. He was regarded as perhaps the fastest skier on the biathlon tour, but his lack of shooting accuracy often prevented him from producing consistent and winning results.