Sielck's first band was Gentry, which he formed along with Kai Hansen. In 1982, he left the band and worked first as a technician, then traveled for one year to Los Angeles. After his return, he began his career as a producer; his first production was Heading for Tomorrow, the debut album of Gamma Ray, the new band of his old friend Hansen. Other bands with which he cooperated are Uriah Heep, Saxon, and Blind Guardian. He maintains a good relationship with many bands, having participated in several of their albums as a guest musician.
In 1996, Sielck created his own band, Iron Savior, together with Hansen and Blind Guardian's then-drummer Thomen Stauch. Those two would step out later, but Sielck continued the band with new musicians.
In 2004, he agreed to produce and participate as a guitarist/bassist in Stauch's new band Savage Circus. He left Savage Circus in December 2011 to focus on Iron Savior exclusively again.