2011-12: Christoffer Ehn skated for the Skovde IK club in western Sweden near his hometown of Skara and played for Vastergotland in the 2012 TV-Pucken tournament for high school players. He scored 4 goals and was +2 with 4 penalty minutes in 14 games for the Skovde IK U20 team. Ehn scored 7 goals with 4 assists and was minus-28 with 16 penalty minutes in 25 games for the Skovde IK U18 team. In eight games for Vastergotland he scored 5 goals with 7 assists and was +13 with 2 penalty minutes.
2012-13: Ehn went from Skovde to the powerful Frolunda club; skating for the U18 squad. He scored 9 goals with 22 assists and was +22 with 16 penalty minutes in 33 regular season games and had 1 goals with 2 penalty minutes and was +1 in three playoff contests.
2013-14: Ehn made his pro debut in the SHL as a 17-year-old — appearing in two February games with Frolunda with no points nor penalties — and spent most of the season with the club’s U20 team. He represented Sweden in international play for the first time — skating for the U18 team at the 2014 World Junior Championship in Finland. In 48 games with the Frolunda U20 team, including three playoff games, he scored 7 goals with 7 assists and was minus-6 with 14 penalty minutes. Ehn scored 8 goals with 10 assists and was +15 with 4 penalty minutes in 15 regular season games for the Frolunda U18 team and had 1 goal with 3 assists, finishing +1, in five playoff games as the club captured a bronze medal. In seven games for fourth-place Sweden at the WJC he had 3 assists and was -1 with no penalties. Ehn was ranked 89th amongst International skaters in the Central Scouting final rankings and was selected by Detroit in the fourth round (106th overall) in the 2014 NHL Draft.