[3] Janssen made her first appearance at international level in 2017 when she won the silver medal with the air rifle at the Junior World Championships in Suhl.

Janssen later spoke of this time as a motivational crisis: she said it took her some effort to train because the forced break made her realize how time-consuming the sport is.

In August of the same year, she also won the bronze medal at the 2023 Shooting World Championships in Baku, this time with the team in the 10 m air rifle competition.

[8] At the time, Janssen was one of the few top female shooters worldwide who did not earn money by working for a government agency such as the police or the military.

She received most of her financial support from her parents' horticultural business and the German Sports Aid Foundation, and she also had local sponsors.

At the Olympic Games, Janssen qualified for the mixed bronze medal match with her fellow marksman Maximillian Ulbrich.