Kleinwächter was born in Chernivtsi, Austria-Hungary (formerly the capital of the historic region of Bukovina; today a city in southwestern Ukraine on the Prut River).

From June 1925 to December 1926, he was counselor in the Austrian legation in Washington, D.C. After returning to Austria, he worked for several years for the Federal Press Service in Vienna, interrupted by a stay in Canada, where he headed the Consulate General in Ottawa from 1930 to 1932 (appointed Consul General, 1st Class, in 1931).

[1] After the Anschluss of Austria to Hitler's Germany, he was dismissed from the diplomatic service and arrested on 12 March 1938 as a "half-Jew".

Because of his experience as a diplomat in America, the Renner Provisional State Government appointed him Permanent Representative of the Ministry of Foreign Affairs to the American delegation of the Allied Commission for Austria.

After the National Council elections in November 1945, the new Federal government under Chancellor Leopold Figl sent him as Envoy Extraordinary and Minister Plenipotentiary to Washington, where he arrived in February 1946.

The writing paper they received in boxes from Vienna dated back to the Nazi era and bore the Swastika, so Kleinwächter and Thalberg burned it in the bathtub of their hotel room as a precaution.

He successfully lobbied U.S. Secretary of State James F. Byrnes to support Austria's request for urgently needed UNRRA aid.

[10][1] Another of Kleinwächter's diplomatic successes was the signing of the first Fulbright Agreement between Austria and the United States in June 1950.