Siemon-Netto began his journalism career 1956 as a trainee at "Westfalenpost", a large regional newspaper in southern Westphalia. In 1958, he joined the Associated Press in Frankfurt first as copy editor, then as slot editor and roving reporter, covering, among other things, the construction of the Berlin Wall in 1961. From 1962 to 1969, he worked as a correspondent for Springer Foreign News Service in London, Paris, New York, Vietnam, the Middle East and Hong Kong. His assignments included the United Nations, the U.S. civil rights movement, the assassination of President John F. Kennedy, the Vietnam War (over a period of five years), the Arab–Israeli Six-Day War, and China's Cultural Revolution. From 1969 to 1973, Siemon-Netto was North American correspondent for the magazine, "Der Stern", writing about many major news events in North, Central, and South America, and in East Asia, France, and again Vietnam.