In 1880 he used forged documents to move to St Petersburg, which was officially out of reach: a rare
exception to the obligation to settle in large cities was made only to "useful Jews", such as registered prostitutes, former cantonists,
or very wealthy merchants (see Pale of Settlement).

Soon Dubnow’s publications appeared in the press, including the leading
Russian–Jewish magazine Voskhod. In 1890, during the
expulsion of Jews from the capital city, Dubnow was forced to leave. He settled in Odessa and continued to publish studies of Jewish life and
history, coming to be regarded an authority in these areas.

Dubnow actively participated in contemporary social and political life in the Russian Empire. He called for modernizing Jewish
education, organizing Jewish self-defense (see Pogrom), and for
equal rights, including the right to vote.

In 1906 he was allowed back to St Petersburg, where he founded
and directed Jewish Literature and Historical-Etnographic society and edited the Jewish Encyclopedia. In the same year, he
founded the Folkspartei (Jewish People’s Party), which
successfully worked for the election of MPs and
municipal councillors in interwar Lithuania and Poland. After 1917
Dubnow became Professor of Jewish history in Petrograd University.

In 1922 he emigrated to Kaunas (Kovno)
and later to Berlin. His magnum
opus was ten volumes of History of the Jewish people, first published in German in 1925–1929.

In August 1933, after Adolf
Hitler came to power, Dubnow moved to Riga, Latvia. Nazi troops occupied Riga in
July 1941, and Dubnow, with thousands of other Jews, was
transferred to the Riga ghetto.
According to the few survivors, Dubnow repeated to ghetto inhabitants: "Yidn,
shreibt un fershreibt" (Yiddish: "Jews,
write and record").

On December 8, 1941, Simon
Dubnow was among thousands of Riga ghetto Jews massacred in the Rumbula forest.