He began teaching at Kunstakademie Düsseldorf and was a student of the Swiss abstract painter Paul Klee who had an important influence on Adler's work.

Adler volunteered for the Polish army during World War II but was later discharged for health reasons, eventually settling in Scotland and then Aldbourne, England.

During the election campaign of July 1932 he published, with a group of leftist artists and intellectuals, an urgent appeal against the policy of the National Socialists and for communism.

In the years that followed, he made numerous journeys to Poland, Italy, Yugoslavia, Czechoslovakia, Romania and the Soviet Union.

[5] In 1937, twenty-five of his works were seized from public collections by the Nazis and four were shown in the Degenerate Art Exhibition in Munich.

[7] Adler died in Whitley Cottage in Aldbourne[8] on 25 April 1949 at the age of 53, and is buried at the Jewish cemetery in Bushey, Hertfordshire.