Career
The Cohn-Vossen transformation is also named for him. He was born in Breslau (then a city in the German Empire. Now Wrocław in Poland).
He wrote a 1924 doctoral dissertation at the University of Breslau (now the University of Wrocław) under the supervision of Adolf Kneser.
He became a professor at the University of Cologne in 1930. He was barred from lecturing in 1933 under Nazi racial legislation, because he was Jewish.
In 1934 he emigrated to the Union of the Soviet Socialist Republics, with some help from Herman Müntz. While there, he taught at Leningrad University.
He died in Moscow from pneumonia.