In 1898, together with his cousin Paul Cassirer, he opened a gallery and bookshop at 35 Viktoriastraße near Kemperplatz, Berlin.
Cassirer moved to Berlin, and he and his cousin Bruno, while still in their mid 20's, opened their gallery on the ground floor of Paul's house in the up-market Viktoriastrasse.
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On close terms with art dealers like Bruno and Paul Cassirer, he became a leading figure in the Berlin art scene before World War I.
On 2 February 1910, 35 of Pellerin's Manets were sold for 1,000,000 francs to a consortium of art dealers that included Bernheim-Jeune, Durand-Ruel, and Paul Cassirer, a move that stirred controversy.
Max Liebermann was the Berlin Secession's first president, and he proposed to the Secession that Paul Cassirer and his cousin Bruno act as business managers.
Johanna initially worked closely with German art dealers and publishers Paul Cassirer and his cousin Bruno to organize exhibitions of Van Gogh's paintings in Berlin and in 1914 to publish the first volume of the Letters to Theo.