Siebel joined the CDU in 1945 and was chairman of the Siegerland district CDU association. After 1945 he was a member of the district council of Siegen. He was a member of the German Bundestag from 1949 to 1961. In parliament he represented the constituency of Siegen-Stadt und -Land, in which he was always directly elected and in 1957 even with an absolute majority of 51.4% of the first votes. From 1953 to 1957 he was deputy chairman of the BundestagCommittee for Postal and Telecommunications Affairs.