Janssen was born in Venlo, Netherlands, and began his football career at Venlosche Boys, his hometown club. He moved on to VVV-Venlo, but his contract expired in 2011 before he could make his debut for the first team.[2]
On 9 June 2011, Janssen signed with regional rivals Fortuna Sittard, and he made his professional debut as a substitute for Tim Dreesen in an Eerste Divisie match against FC Oss on 23 September 2011.[3] A few months later, in December 2011, he signed a contract with the club until 2013, with an option for another year.[4] Janssen made eight appearances for Fortuna in his first season, but in the 2012–13 season, he became a regular starter and remained so throughout the season. His contract was extended for two more years in 2014.[5] In January 2015, Janssen, now the team captain, was very productive, scoring the winning goals in games against MVV and Helmond Sport, and even scoring a hat-trick against rivals Roda JC.[6] He finished the 2014–15 season with 10 goals in 36 games.[7]
In June 2016, Janssen returned to his former club, VVV-Venlo, on a two-year contract.[8] He became a key player in defense for VVV and helped them to promotion to the Eredivisie in his first season back, as the club won the second division title.[9] His contract was extended for two more years in June 2018.[10] However, in 2020, his contract with VVV was terminated.[11]
On 21 April 2020, Janssen signed a three-year contract with his former club, Fortuna Sittard, as announced by the club.[12]
On 10 August 2023, Janssen returned to VVV-Venlo on a one-year deal.[13]