After the war, Wischnewski joined the SPD. He was employed by a metal-working company and from 1952 trained as an IG Metall union secretary. Upon the 1957 federal election, he became a member of the Bundestag parliament and also a SPD board member in the Cologne district. He was elected federal chairman of the party's Young Socialists youth organisation in 1959 and joined the SPD federal committee in 1970, from 1979 as deputy chairman. From 1961 to 1965 he also was an elected member of the European Parliament.
Because of his good relations with Arab politicians since the days of the Algerian War, Wischnewski was nicknamed "Ben Wisch" by Chancellor Willy Brandt (later sometimes spoofed as "Ben Cash" because of his duty as the SPD federal treasurer). His efforts decisively improved the West German diplomatic relations with numerous Arab and African countries. Also, after the 1973 Chilean coup d'état, he played a vital role in rescuing German and European hostages.
Wischnewski is best known outside Germany for his involvement in the "German Autumn" of 1977: when the German business executive Hanns Martin Schleyer was kidnapped by the militant Red Army Faction (RAF) in September, Wischnewski visited several Arabian governments and during the joint RAF and PFLP hijacking of Lufthansa Flight 181 in October followed the plane to Mogadishu, negotiating with the Somalian government. His success allowed German GSG 9 troops to storm the plane and rescue 90 hostages; three of the hijackers were killed in the process.