Müller was born in Nuremberg, Germany in 1868. Sometime before 1911, he came to Mexico to prospect for silver and by 1911 he was residing in Chuparosa.
Müller appeared to have a very short fuse and showed a strong dislike for Americans in particular.
John Marston meets Müller in 1911 when he joins in a Poker game which Müller is playing with Landon Ricketts, Manolo Santander and The Stranger, at the Saloon in Chuparosa. After a few rounds, with Müller making comments hinting that he believes John and Ricketts are colluding, John glances over at Müller and then looks down at his own cards. Upon seeing this, the German accuses John of looking at his cards, calling John "a fucking cheat", which John denies. The situation escalates and the players all eventually draw their guns on each other. Müller decides that he and John will Duel for the chips in the pot, with The Stranger and Ricketts as their seconds. Ultimately, John ends up the winner and Müller is shot dead. John and Ricketts then go to have a drink, with Ricketts saying that "Mr. Müller is buying".