Stegner was born in Bad Dürkheim, Germany on October 2, 1959, and received his high school diploma in 1978.

After Simonis's attempt to form a new one-seat majority red-green government supported by a regional party had failed because throughout four secret ballots one representative had not voted for her, she withdrew from politics and Stegner practically took over from her and became Deputy Minister-President and State Minister for Home Affairs in the subsequent CDU-SPD grand coalition led by Minister-President Peter Harry Carstensen.

Although Stegner had harshly criticised the unknown person who had refused to vote for Simonis, he was initially considered to have been the so-called "Heide murderer" by some media, which is generally regarded as refuted because Simonis suspected another person and had planned to pass her office on to Stegner after two years.

In the negotiations to form a Grand Coalition of Chancellor Angela Merkel's Christian Democrats (CDU together with the Bavarian CSU) and the SPD following the 2013 federal elections, he was part of the SPD delegation in the working group on education and research policy, led by Johanna Wanka and Doris Ahnen.

At the SPD national convention in 2014, party chairman Sigmar Gabriel nominated Stegner as one of his six deputies, alongside Hannelore Kraft, Olaf Scholz, Manuela Schwesig, Thorsten Schäfer-Gümbel and Aydan Özoğuz.

[5] Saskia Esken and Norbert Walter-Borjans won the nomination and Stegner retired from his position of deputy chairman.