Kleinmann was born in Barmen (today, part of Wuppertal) in 1876 and studied civil engineering at the Technische Hochschulen in Charlottenburg and Hanover between 1896 and 1900.

During the First World War, he served with the 8th Rhenish Pioneer Battalion and was deployed on the eastern front, from 1916 as head of operations at Military Railway Directorate 9 in Bucharest.

[1] After the war, Kleinmann between 1919 and 1920 was on special assignment in Moscow and St. Petersburg and then in the German legation in Vilnius attempting to secure railroad materials left behind in Lithuania and Latvia.

The Reichsbahn leadership was expected to implement the policies of Adolf Hitler with regard to the dismissal of Jews and Social Democrats, and the filling of key positions with reliable Nazis.

At the same time, he was appointed to the General Council of Göring's Four-Year Plan and served as the leader of its transport work group.

In May 1942, Albert Speer, the recently appointed Reich Minister of Armaments and War Production, was determined to improve the situation and approached Hitler about replacing the 65-year-old Kleinmann with Albert Ganzenmüller, a young 37-year-old railway inspector who recently had successfully restored rail traffic between Minsk and Smolensk.

[4] Kleinmann was then made General Director of Mitropa, a catering company that managed dining and sleeping cars, where he would serve out the rest of the war.

[5] At the end of the war in Europe in May 1945, Kleinmann was thought to have been seized in Berlin by the Red Army and taken to an internment camp near Posen (today, Poznań).