During his first years, Northug was generally considered a star of the future. Whilst he was still a junior Verdens Gang newspaper reported in November 2005 that "the ski manufacturers are fighting to get Northug". Fischer won his signature, and his contract included a base salary (the first time ever for a junior), which would be multiplied by five if he made the Norwegian elite team for 2006/07, which he did. The contract also offered bonuses if he won medals in the junior World Championships.
Northug has six gold medals from junior World Championships. His first two gold medals came in 2004/05 in the pursuit and the 10 km freestyle in Rovaniemi, then in 2005/06 in Kranj he won gold in the 10 km classic, the pursuit, the sprint, and the relay. He also has two silver medals (one in the sprint, and one in the relay, both in 2004/05). His victories made him the first athlete ever to win five individual gold medals at the FIS Junior Nordic World Ski Championships. During 2005/06 he also took part in the Norwegian National Championships, and won the double pursuit race, beating Frode Estil by 1.9 seconds, and became the first junior to ever win a Norwegian National Championship.
Before he joined the World Cup he competed in a few Continental Cup meetings, but mostly in the Scandinavian Cup. During his Scandinavian Cup career (2004/05 & 2005/06) he had seven podium finishes, he came second once in 2004/05, and in 2005/06 he scored four victories, and two second places.
The 2005/06 season was Northug's first in the World Cup, although he competed in one race the season before, a sprint in Drammen, he came 35th. During the 2005/06 season he shared his time equally between the World Cup and the Scandinavian Cup, although in early May 2006 it was announced that Northug would be in the senior national team for the 2006/07 season. Northug also claimed his first World Cup victory in the 2005/06 season, in a pursuit race in Falun, beating 2005/06 World Cup winner Tobias Angerer, who came second, and 2004/05 overall champion Axel Teichmann was third. Then in the last race of the year, a pursuit in Sapporo he claimed another podium place, coming second. He lost to Mathias Fredriksson by 3.8 seconds. He also came seventh, tenth, and twelfth in sprint races, and fifteenth in the 50 km freestyle in Holmenkollen. Northug finished the 2005/06 World Cup season in 14th place overall. He also finished 14th in the distance standings, and 24th in the sprint.
After Norway's disappointing display at the 2006 Winter Olympics, the Norwegian press questioned why Northug was not taken to the Games. Northug himself admitted he was disappointed after not getting selected, especially as he had won the double pursuit in the National Championships earlier in the year. The day after the 2006 Olympics Team was announced, Northug was on the team winning the Norwegian Championships in 4x10 kilometer relay. As he crossed the finish line, he shouted "And I am not going to the Olympics?".
Northug won his first gold medal at the FIS Nordic World Ski Championships in Sapporo as a member of the 4 x 10 km relay in 2007. Northug completed the last leg of the relay, beating Sweden and Russia on the sprint for the finish line. He was also in a good position to compete for silverware in the 30 kilometer duathlon, but he fell in the last part of the race and was disappointed to finish fifth.
He was far more successful at the FIS Nordic World Ski Championships 2009 in Liberec, where he earned three golds in the 15 km + 15 km double pursuit, 4 x 10 km relay and 50 km freestyle mass start. In all three events he sprinted away from the rest of the pack to win the race.
Northug was the runner up to the overall World Cup in the 2008/2009 season, losing to the Swiss Dario Cologna after leading before the final races.
Northug finished in an extremely disappointing 41st place in the first Cross Country event during the 2010 Winter Olympics in Vancouver, Canada. A day later, Northug responded with a bronze medal in the Sprint Event. This success was short-lived however, as he broke his pole in the end of the 30 km pursuit, where he was one of the biggest favourites to win. He then won his first Olympic gold, in the Team Sprint, alongside Øystein Pettersen.
Days later he was skiing the anchor leg in the 4 x 10 km relay. When he took over from Lars Berger who was skiing the 3rd leg, he was 37.5 seconds behind the lead group. Despite this he managed to catch and overtake France and the Czech Republic to win Norway a silver medal. Northug then won his first individual medal at the olympics when he won gold at the Men's 50 kilometre classic. Less than two weeks later, he won the 50 kilometre freestyle event at the Holmenkollen, becoming the first skier to win the 50 km at the Winter Olympics, World Championships, and Holmenkollen since Sweden's Gunde Svan reached that triple crown in 1988.
In September 2010 details of Northug's sponsorship contract with soft drink manufacturer Red Bull were publicized by Norwegian broadcaster TV 2, which did not divulge its sources, revealing the most profitable sponsorship agreement with an individual athlete in Norwegian history. For four years, until after the 2014 Winter Olympics, Northug will be receiving a minimum of NOK 1 million annually with a prospect of getting twice the amount if his performances equal those in his most recent seasons.
Northug did not start the 2010–2011 season well, missing the first three weeks of the world cup due illness. His Tour de Ski campaign was also marred by poor results (to his standard) in the opening stages. He however climbed the rankings and finished 2nd after winning the prestigious penultimate stage (20 km classical mass start) in Val di Fiemme (taking all intermediate bonus sprints as well which has never been done by the winner of the race). In the Holmenkollen World Ski Championship Northug raced in five disciplines, taking 3 gold and 2 silver medals. During the 2011 World Championship, he gained widespread international attention when he controversially crossed the finish-line sideways after convincingly beating his opponents on the last leg of the 4x10 kilometre relay. The gesture was regarded by media as disrespectful, most notably towards his most fierce rival, Marcus Hellner. At the end of the season Northug also won the Season Finale in Sweden, beating his compatriot Finn Haagen Krogh.
In the FIS Nordic World Ski Championships 2013, Northug won the 15 km freestyle for the first time at a World Championships, completing his collection of World Championship gold medals. He again beat the Swedes on the sprint of the 4x10 km relay, securing Norway's 7th World Championship gold medal in a row in the relay. Northug also won a silver in the individual sprint. He was less successful on the team sprint, where he and his teammate, Pål Golberg, went out in the semifinal. Despite being a favorite before the 50 km, Northug finished 21st. He gave the winner, Johan Olsson, his jersey after the race as a gesture of respect.
Northug had an impressive ending to the 2012–13 season. He won 15 km classical individual in Lahti by over half a minute before being victorious in the famous classical sprint in Drammen. In the Season Finale in Sweden, Northug won first two stages, the prestigious classical sprint in Stockholm and free style prologue in Falun, then finished fourth in the penultimate stage, 15k classical mass start, and closed the season by winning the whole mini-tour. Before Lahti, Northug was in the third place in the world cup, 226 points behind the lead; but he finished 180 points ahead of the second place.
The 2013-14 season saw Northug struggle with an illness which severally impacted his performance. Particularly frustrating for him was his inability to fight for medals in the skiathlon and individual sprint of the Sochi Olympics. His lackluster performance by his standards in the 50 km event, which saw him finish a distant 18th in a race he had convincingly won four years earlier, meant he left a major world event without a single medal for the first time in eight years.
Northug showed better physical shape in the 2014/2015 season, and proved to be Sundby's strongest competition during that season's Tour de Ski. Despite leading before the final climb up the Alpe Cermis, he lost to Sundby, who won his second TdS title in a row. In July 2016, Sundby lost that title to Northug, after Sundby's anti-doping rule violation, thereby giving Northug his first Tour de Ski win.
At the 2015 World Championships in Falun, Petter secured his 10th WC gold medal by narrowly beating Canada's Alex Harvey in a bunch sprint to the finish line in the men's classic sprint event. He then proceeded to contribute to Norway's victory in both the team sprint and 4x10km relay. The final race of these World Championships, i.e. the 50 km classical, saw Northug score one of his greatest victories. A victory which left him speechless at the after race press conference, a premiere for the usually voluble Petter. The race was held in difficult conditions due to heavy snowfalls which prevented any breakaway and saw the main favorites fight a tactical battle from start to finish. In the last climb before the finish Northug managed to slot into 3rd to fight for victory in the last dash to the finish line. His now legendary double poling was in full display as he managed to overtake his remaining two opponents to capture one of the most coveted prizes in cross-country skiing.
As of 2015, Northug is unmatched as to number of medals among active cross-country skiers. His tally of 20 Olympic and World Championship medals (15 gold, 4 silver, 1 bronze) stands high above his peers.
He has two brothers, Even and Tomas. Tomas Northug also skis professionally and won the Junior World Championships in sprint in 2010. Northug finished school in 2006, but during the summer of 2005 he changed schools from Steinkjer to Meråker and moved into a cabin next to the ski trails to optimize his training conditions.
Petter Northug is an active poker player, and has openly supported the legalization of poker in Norway, which is currently illegal when prizes are involved under Norwegian gambling laws. He participated and cashed (on 653rd place) in the 2010 World Series of Poker main event which he played with skiing rival Marcus Hellner.
Petter Northug has his own music video called "Rise Again" produced by Jack Taylor and featuring Mo of Norway
In the early hours of 4 May 2014, Northug crashed his car near his home in Byåsen, Trondheim, while driving under the influence of alcohol. It was at the time suspected that the car was driving way over the speed limit, which was 40 km/h at the site of the crash. A 23-year-old male passenger broke his collar bone in the crash. After the crash, Northug fled the scene, but was located at his nearby residence by the police using search dogs. In a press release made later the day of the crash, Northug apologised for the incident.
On 15 September 2014, Northug was charged by the Norwegian public prosecutor on five counts of violation of the Norwegian Road Traffic Act and one count of violation of the general civil penal code. The latter charge stemmed from Northug having attempted in three separate police interrogations on the day of the crash to blame his passenger for the incident, claiming that the passenger had been driver of the vehicle. Due to Northug's initial claims, the passenger had been preliminary charged with causing the incident.
Because Northug pleaded guilty to all the charges leveled against him, he received a summary trial, which took place on 9 October 2014 at the Sør-Trøndelag District Court in Trondheim. Northug was sentenced to 50 days of prison, ten days less than requested by the prosecutor. He had already served two days of the sentence in police custody after his initial arrest. In addition to the prison time, Northug was fined 185,000 kr and lost his driver's license for five years. Northug accepted the verdict and waived his right to appeal.