Paul Cassirer started out as a student of art history, and then became a writer in 1890s Munich, where he worked for the weekly magazine Simplicissimus and published two novels.
Cassirer moved to Berlin, and he and his cousin Bruno, while still in their mid 20s, opened their gallery on the ground floor of Paul's house in the up-market Viktoriastrasse.
His father, Louis, was an engineer and businessman, whose company — Kabelwerke Dr. Cassirer & Co. — manufactured telegraphic cables, and was eventually taken over by Siemens.
In 1910 he married Ottilie Godefroy (18 August 1880, Vienna – 21 February 1971, Berlin), the actress well known under her pseudonym Tilla Durieux.
On April 11, 1900, Paul Cassirer purchased "Rue St. Honoré, après midi, effet de pluie", by French Impressionist Camille Pissarro (the “Painting”).