Decker was born in Hungary to a German-speaking family, and grew up and made his career in Vienna, where he taught painting as well as working as a portrait and historical artist.
He was born in Pest, in the Kingdom of Hungary, but in 1821 the Decker family moved to the imperial city of Vienna, where he grew up and was taught to draw and paint in watercolour and miniature by his father.
After that, Decker painted portraits in oils, and then from the 1850s also in pastels, having studied the work of Mengs and Liotard in Dresden and been captivated by the medium.
[5] Outside the world of art, Decker was an active member of the Vienna Chess Company (Wiener Schachgesellschaft) while Albert Salomon Anselm von Rothschild was its driving force.
[6] On his son's name day on 17 April 1869, the Emperor Franz Joseph presented the ten-year old Rudolf, Crown Prince of Austria, with a portrait of Archduke Charles, Duke of Teschen, victor of Aspern, by Decker.