He worked as a financial agent for a local Lithuanian magnate, the Lord of Kretinga and Bishop of Vilnius Prince Massalski.

He spent some time in Paris during the beginning of the French Revolution, and it is thought that this may have later inspired him to join Tadeusz Kościuszko, who advocated similar causes of brotherhood and equality.

Joselewicz, along with another Jew named Józef Aronowicz, issued a patriotic call-to-arms in Yiddish denouncing Russia and Prussia, eliciting hundreds of volunteers, mostly poor trade workers and artisans.

At Joselewicz's request, they were allowed to keep their religious customs, including access to kosher foods, abstaining from combat on the Sabbath when possible, and growing their beards.

He was killed in the Battle of Kock in 1809 during an encounter with a unit of Austrian hussars, and today his grave has become a popular tourist attraction.

Although Berek is most famous for his service for Poland, in 1796 he proposed to the Habsburg emperor the raising of a corps of six thousand to eight thousand Jews who would be divided into cavalry and infantry units to fight against the French.” [3] There are streets named after him in many Polish towns including Warsaw, Kraków, Łódź, Częstochowa, Zawichost, Biłgoraj, Radomsko, Chrzanów, Będzin, Lubartów and Żarki.

Berek's son, Józef Berkowicz (1789–1846), also fought in the Battle of Kock, and later served as a squadron chief during the November Uprising of 1830, during which he also attempted to convince Jewish soldiers to desert the Russian army and join the Poles.