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.