He obtained his doctorate in 1934 and worked in the legal department of the Chamber of Labour which at that time was integrated into the Austrofascist unitary trade union centres by the government of the Federal State of Austria.

[3] During World War II he served in the German Wehrmacht, temporarily as a staff member for General Heinz Guderian, as well as in campaigns in Poland, France, Finland and Russia.

When his party colleague, Chancellor Julius Raab, finally resigned in 1961, Klaus' influence as a representative of the "young reformers" grew.

In office from 2 April 1964, Klaus initially continued the grand coalition with the Socialists under Vice-Chancellor Bruno Pittermann according to the Proporz system that had governed Austria since 1945.

In September 1971 he published his memoirs "Macht und Ohnmacht in Österreich", and up to 1995 he frequently led seminars on political and social themes.