He was the son of district attorney and Copenhagen’s chief constable Aage Valdemar Seidenfaden (1877–1966) and Anna Elise Reenberg Teilman Harck (1887–1928).
When the Spanish Civil War broke out in July 1936, he compiled reports from the front on the government's side, in 1937 publishing the book Borgerkrig i Spanien.
His militant attitude towards the Nazi threat often clashed with that of the influential social liberal Det Radikale Venstre party and the views of the several times foreign minister Erik Scavenius (1877–1962), and chairman of the board of Politiken.
[5] [6][7] When Denmark was invaded on 9 April 1940, Erik Seidenfaden was in Oslo reporting on the Allied minefield laying operations in Norwegian waters.
He told the editor in chief of Politiken that he had joined the News Chronicle, and agreed to stay only after being offered a post as a correspondent in Rome to cover neutral Italy.
After Italy entered the war in early June 1940 he returned to Denmark and began becoming active in illegal circles which, apart from Nicolaj Blædel, included the journalists Merete Bonnesen (1901-1980), Jens Søltoft-Jensen (1906-1964), Sten Gudme (1901-1961), the Gyldendal publisher Ingeborg Andersen (1887-1960), the philologist Lis Jacobsen (1882-1961), the physicist Niels Bohr (1885-1962) and his brother mathematician Harald Bohr (1887-1951), the Politiken board member Herman Dedichen (1896-1958) and the jura professor Stephan Hurwitz (1901-1981).
[8] When the British military intelligence organization SOE-Special Operations Executive in their first mission in Denmark dropped parachutists Carl Johan Bruhn (1904–1941) and Mogens Hammer (1911–1946) by Haslev on 27 December 1941, they were given two contacts who, it was believed, would be prepared to assist them.
In Stockholm, he was soon active in establishing a network of underground news informers in Denmark, with the help of his brother Gunnar Seidenfaden who worked in the Foreign Ministry, and his father, who at the time was the chief constable of Copenhagen's northern district.
In Stockholm, he forged close ties to the SOE-Special Operations Executive chief Ronald Turnbull (1914–2004) and became an SOE agent n.4388 with the rank of officer.
The DPT service was particularly useful during the general strike in Copenhagen in July 1944, when all power was cut, nor the local press nor the Danish radio operated.
Information and Erik Seidenfaden probably exerted the strongest influence in the fields of defence and foreign policy, and his campaign for Denmark joining NATO in 1949 had crucial impact.
Indeed, when its reporter Elsa Gress followed very critically phenomenon of McCarthysm in 1952 and 1954, Erik Seidenfaden gave a very unflattering presentation of McCarthy's career and even concluded by expressing the pious hope that somebody would eliminate the senator.