2010-11: Nikolaj Ehlers skated in the Swiss Elite Novizen U17 league as a 14-year-old. In 28 regular season games for Biel he scored 19 goals with 11 assists and had 8 penalty minutes. Biel avoided relegation to the second division. Ehlers scored 7 goals with 9 assists and 6 penalty minutes in seven relegation playoff games.
2011-12: Ehlers was the third-leading scorer for Biel's U20 team despite skating in just 28 of 38 regular season games — and skated for Biel's U17 team and Denmark's U16 squad. He scored 17 goals with 17 assists and had 6 penalty minutes in Elite Jr A play. In four relegation playoff games he scored 2 goals with 2 assists and 2 penalty minutes. Ehlers scored 26 goals with 20 assists and 14 penalty minutes in 35 games with Biel's U17 team. He scored 1 goal with 1 assist and 4 penalty minutes in three international U16 matches for Denmark.
2012-13: Ehlers played 18 games for Biel's men's team in the Swiss National A League (including seven playoff games) — spending most of the year with the club's junior team — and was part of Denmark's gold medal-winning U18 squad at the 2013 U18 Division 1A World Junior Championship. He scored his first professional goal and had one assist; finishing +1 in 11 regular season games with Biel. In seven playoff games he was an even plus/minus with 3 assists. In 34 games with Biel's U20 squad he scored 30 goals with 23 assists and had 36 penalty minutes. Ehlers scored 3 goals with 8 assists and was +3 with 2 penalty minutes for Denmark at the WJC D1A tournament. He was selected by Halifax in the first round (6th overall) of the 2013 CHL Import Draft and was chosen by Avangard Omsk in the fourth round (127th overall) of the 2013 KHL Draft.
2013-14: Ehlers made a smooth transition from Europe to North America — proving to be an offensive force for Halifax in his first QMJHL season — and in December was part of Denmark's gold medal-winning U20 squad at the Division 1A World Junior Championship in Poland. He scored 2 goals with 4 assists and was +6 with 4 penalty minutes in five games for Denmark. Ehlers was selected to play for Team Orr in the 2014 CHL/NHL Top Prospects Game.