[1] Christiansen began his career by racing in biathlon and cross-country skiing as a junior on the national level.

On the last day of the 2004 World Championships, Christiansen set a long-term goal of qualifying for the 2014 Winter Olympics.

[3][4] Before turning 17, Christiansen had made a clean sweep at the Junior Norwegian Championships in his age group, winning four gold medals and the Norwegian Biathlon Cup (Norgescupen) in his age category, called the Statkraft Junior Cup overall.

[5][6] He had also won two golds in the Junior Norwegian Summer Biathlon Championships in September 2008, in the 17-year-olds age group.

[10][11] This caused Christiansen to be selected for the first team of Buskerud alongside Christian Georg Bache, Frode Andresen and Ole Einar Bjørndalen.

Though he incurred a penalty loop, he made up two places over his leg, and good work by Andresen and Bjørndalen secured the gold.

He changed in third, but a total of five penalty loops by his teammates, Anders Brun Hennum, Bache and Andresen, meant they finished in fifth.

[29] Two days later, Christiansen raced in the 12.5 km individual in which he incurred three penalty minutes, and yet won bronze.

Fast skiing and good shooting by his teammates, Andresen, Hennum, and the returning Bjørndalen, meant that the team from Buskerud again won gold, more than two minutes ahead of second place.

In the first event, the 10 km sprint on 27 January, he missed once but still won gold, just one second ahead of Alexandr Loginov.

The Norwegians led at the last exchange, and as Christiansen needed only two spare rounds to clear the targets, they won gold.

[48] Christiansen skied better in the pursuit the following day, where he missed twice but climbed eleven places to win silver just ahead of Alexandr Loginov, and over one minute and forty seconds behind the sprint winner Tsvetkov.

[49] In March, Christiansen raced in the Junior Norwegian Championships, winning silver in the individual and gold in the sprint and finishing fourth in the relay.

[56] In September 2012, Christiansen competed at the Norwegian Summer Biathlon Championships in Holmenkollen for the first time in the senior group.

[59] The race was the 10 km sprint, and he missed twice and finished 27th, 1 minute and 18 seconds behind the winner, the Canadian Jean-Philippe Leguellec, qualifying for the pursuit the following day.

Birkeland shot fast and cleanly in the first leg to hand over to Bjørndalen just 9.8 seconds behind, in fourth place.

Bjørndalen would require only one spare round, which, combined with his swift shooting and skiing, brought the Norwegians the lead.

Christiansen skied fast and shot cleanly, and this, combined with mistakes by other teams, increased the lead to 47.0 seconds.

[66] The next race, the 10 km sprint, was postponed twice due to heavy fog, giving the Norwegians two extra days to acclimatize.

Christiansen won silver in the pursuit, having missed three times, the same amount as the winner, Benedikt Doll.

[69] The second barrier Christiansen broke in the World Cup was finishing in the top ten in an individual race.

Christiansen shot cleanly and finished tenth, one minute and four seconds behind the winner, Martin Fourcade.

[70] This result, alongside the 11th-place finish in the pursuit, qualified Christiansen for his first mass start in the World Cup, breaking another barrier.

[80][81][82] Christiansen helped the Norwegians finish in second place in the opening mixed relay of the season in Östersund.

However, Johannes Thingnes Bø incurred three penalty loops on the first leg and changed to his brother in 22nd place.

[91] In the following 20 km individual, Christiansen shot well but skied rather slowly and finished 56th and decided not to start in the pursuit.

[59] In Pokljuka, in the first World Cup round after the Olympics, Christiansen finished 64th in the 10 km sprint and thus did not qualify for the pursuit.

[97][98][99] In the relay, the Buskerud team suffered in the absence of the ill Bjørndalen and the retired Andresen, and finished sixth.

[116] Christiansen finished 67th in the sprint and decided to return to Norway for a medical examination, having been feeling tired for some time.