Coming from Graz:
take the A9 Phyrnautobahn north, then S35 Brucker Schnellstraße and S6 Semmering Schnellstraße to Kapfenberg exit, then B20 (Mariazeller Bundesstraße) 49 km north (signpost). Variant: S6 to exit St. Marein and then over the Pogusch to Turnau and from Au on the B20 to Mariazell and then on to St. Sebastian
Coming from Salzburg: take the Westautobahn A1 to exit Ybbs, then B25 to Scheibbs, from there two options: either B28 via Wastl am Wald to B20, then via Annaberg and Josefsberg or B25 further to Grubberg and then B71 via the newly developed Zellerrain, turn left in St. Sebastian and take B20 towards Mitterbach to the airport.
Coming from Vienna: take the A1 Westautobahn to the St. Pölten Süd exit, then the B20 (Mariazeller Bundesstraße) 76 km south, either via Annaberg and Josefsberg or in Freiland turn left from the B20 onto the B21 and continue via Hohenberg and St. Aegyd through the Halltal valley to Mariazell (sufficiently signposted).
Variant: Südautobahn A2 and S6 Semmering expressway to Mürzzuschlag, then via Neuberg and Mürzsteg on the B21 to Mariazell.With both variants then from St. Sebastian on the B20 direction Mitterbach to the airfield.
Detailed information on how to get there can be found here.