In 2005 he won the provincial schoolboy championship playing for his high school, Sir Oliver Mowat Collegiate Institute.

They played in five slams that season, making it to the semifinals of the 2014 Masters and 2015 Players' Championship, while missing the playoffs at the November 2014 National, the 2014 Canadian Open of Curling and the 2015 Elite 10.

The team played in the 2015 Ontario Tankard (men's provincials), making it to the final before losing to Janssen's former skip, Mark Kean.

In the 2016–17 season, the Epping rink again played in all seven slams, making it to the semifinals of the 2017 Humpty's Champions Cup, the quarterfinals of the 2016 WFG Masters and the 2016 Boost National, and missed the playoffs at the 2016 GSOC Tour Challenge, 2017 Meridian Canadian Open, the 2017 Elite 10 and the 2017 Players' Championship.

They represented Ontario at the 2018 Tim Hortons Brier, where Epping led them to a bronze medal, after losing in the semifinal to Alberta, skipped by Brendan Bottcher.

For the 2018–19 season, Janssen joined the team skipped by Charley Thomas as second, with Adam Casey as third and Fraser Reid as lead.

He also spared for Bottcher and represented Alberta at the 2020 Tim Hortons Brier in his home province of Kingston, Ontario where the team won a silver medal.

In the championship pool they improved their record to 9–3, earning the third seed in the playoffs where they defeated Matt Dunstone's Team Saskatchewan in the semifinals to make it to their fourth Brier finals in a row.