[1][2] Decker was born in Cologne, Germany, where he studied at the Hochschule für Musik with Philip Jarnach and Eugen Papst.
[3] He made his conducting debut at the age of 22 at the Cologne Opera, and four years later was appointed to the Staatsoper Wiesbaden and subsequently to the positions of conductor of the Wiesbaden Symphony Orchestra and Generalmusikdirecktor in Bochum.
In 1948, Decker was introduced to the composer Richard Strauss at a card game of whist.
Decker was highly regarded for his performances of Richard Wagner, Richard Strauss, Anton Bruckner, Max Reger and Gustav Mahler.
Among the soloists he collaborated with were Arthur Rubinstein, Emil Gilels, Shura Cherkassky, Clara Haskil, Ida Haendel, Martha Argerich, Hélène Grimaud, Elisabeth Schwarzkopf, Jessye Norman, Dame Kiri Te Kanawa, and Jon Vickers.