In his youth, Janssen played for SV Leones from Beneden-Leeuwen, where he made his debut in the first team at the age of seventeen.[1] Janssen played professional football from 1982 to 2000 with, in chronological order, NEC, Fortuna Sittard, PSV, Kortrijk, again Fortuna and finally again with NEC. His greatest personal achievement was the victory of the Europa Cup I with PSV in 1988 when he scored the decisive penalty-kick in the final against Benfica, after coming on as a substitute.[2]
^"Historie Sportvereniging Leones, Beneden-Leeuwen". sv-leones.nl (in Dutch). sv Leones. Archived from the original on 22 October 2013. Retrieved 29 September 2020. De bekendste speler die bij Leones is begonnen is ongetwijfeld Anton Janssen. Hij debuteerde op 17-jarige leeftijd in Leones 1 om vervolgens de overstap naar N.E.C. te maken.