Lerjen made his debut on the Kreischberg in January 2011 in the Freestyle Skiing World Cup and finished 24th in the halfpipe. In the 2011/12 season he finished second in slopestyle at the Swiss Freeski Open in Davos and at Engadinsnow in St. Moritz . In the 2013/14 season he came under the top teeth twice in the World Cup. He achieved his best placement in the World Cup with fourth place in the halfpipe in Calgary in January 2014 and finished eighth in the halfpipe World Cup at the end of the season. In February 2014 he took part in the Winter Olympics in Sochi for Switzerland. In qualification he was 14th in the halfpipe and thus missed the finals. The following year he won the bronze medal in the halfpipe at the 2015 Freestyle Skiing World Championships on Kreischberg. At the 2015 Swiss Championships in Corvatsch , he finished second in the halfpipe. After taking a break in the 2015/16 season due to a shoulder injury, he ended his career in May 2016.