He grew up in Franconia, and went on to study law in Erlangen, Saarbrücken and Cologne. In 1965, he earned a doctorate with the thesis Verfassunggebung und verfassunggebende Gewalt des Volkes. Subsequently, he earned his Habilitation in 1972, with the thesis Öffentliche Verwaltung durch Private.
In 1973, Steiner was appointed as Professor of Public Law at the University of Erlangen, and subsequently served as Professor at the universities of Göttingen, Bielefeld and Regensburg. He was Dean of the Faculty of Law at the University of Bielefeld from 1976 to 1977, and in Regensburg from 1988 to 1990. Between 1976 and 1979, he also served as a Judge at the Oberverwaltungsgericht.
He was appointed a Judge at the Federal Constitutional Court in October 1995, and still lectures at the University of Regensburg. He was retired as a Judge upon turning 68 in 2007, and was succeeded by Ferdinand Kirchhof.
Steiner married in 1967 and has four children.