He was a railroad worker and journalist who publicized Naziatrocities through the underground Communistpress and fought against National Socialism in the German Resistance. He was a key member of the Red Orchestra.
He returned to the Detroit and met his future wife, Sophie, in 1924, while working as a college intern. He stayed in the United States until February 1928, when Sieg and his wife returned to Germany and he became a freelance author in Berlin He began writing articles for Die Tat, a newspaper published by Adam Kuckhoff. After joining the Communist Party of Germany that same year, he began to write for the arts section of the KPD newspaper, Die Rote Fahne and he got to know Wilhelm Guddorf and Martin Weise.
He was arrested by the Sturmabteilung (storm troopers) in March 1933 and held till June. Upon his release, he began working with the Communist Resistance in the Berlin suburb of Neukölln, becoming the focal point of several groups. He had close contact with Arvid Harnack and Kuckhoff. He took part in leafletting campaigns and shared political information. In 1937, he got a job with the Deutsche Reichsbahn, eventually working as a signaller at the S-Bahn station at Papestraße. As a railroad employee, Sieg was able to make use of work-related travel and free travel to build connections with other Resistance groups, such as the one organized around Bernhard Bästlein.
He worked with Herbert Grasse, Otto Grabowski and the Saefkow-Jacob-Bästlein Organization to produce the newspaper, Die Innere Front (The Internal Front). He was a core member of the Rote Kapelle, along with Guddorf and Kuckhoff.
He was arrested on October 11, 1942 and was taken to the Gestapo prison on Prinz-Albrecht-Straße, where he endured intensive interrogations and abuse. The previous spring, he had confided to a friend that if he were ever arrested, he would commit suicide rather than risk betraying friends. On October 15, 1942, following severe mistreatment, he hanged himself in his cell