According to most sources, Berek Joselewicz was born in Kretinga, Lithuania around 1764. After receiving a traditional Jewish education, he served as a faktor (broker/agent) and arendarz (leaseholder) for the Polish magnate Ignacy Massalski. His work involved frequent travels and extended stays around Europe, and he is reported to have learned French while staying in Paris around the time of the French Revolution. Around 1788, he moved to the Praga neighborhood of Warsaw, Poland with his wife, Rokhla, and became involved in supplying the army.

Joselewicz's experience with the French Revolution and the similarity in his views on equality with that of Tadeusz Kościuszko are thought to have inspired him to participate in Kościuszko's Uprising in late 1793. In 1794, he petitioned Kościuszko for the formation of an all-Jewish cavalry unit, known as the Pułk Lekkokonny Starozakonny (Orthodox Jewish Light Horse Regiment). After the regiment was officially announced, Joselewicz and another Jew, Josef Aronowicz, issued a call-to-arms in Yiddish that elicited more than 500 Jewish volunteers. At the request of Joselewicz the commander, men in the all-Jewish cavalry unit were allowed to practice religious customs, including accessing kosher foods, abstaining from combat on the Sabbath when possible, and growing their beards.

Unfortunately, the regiment saw terrible defeat during the Battle of Praga, where Joselewicz was one of the few cavalry members to survive. The fallen cavalry members were reportedly buried in the Jewish Cemetery in Bródno.

After his defeat, Joselewicz moved to Lwow and attempted to form an all Jewish volunteer troop within the framework of the Austrian army, but his proposal was rejected. In 1797, he traveled to Italy to join the Polish Legions under Henryk Dąbrowski, and saw combat in several battles during the War of the Second Coalition.

During the Napoleonic Wars, Joselewicz commanded a sabre company of Polish cavalry units fighting alongside French forces. With the formation of the Duchy of Warsaw in 1807, he joined the Army of the Grand Duchy of Warsaw as a squadron leader of the 5^th Mounted Rifle Regiment. In early 1809, Joselewicz's squadron was tasked with clearing isolated regiments of Austrian troops and capturing crossings over the Wieprz River in the name of the Duchy of Warsaw and its allies.

During clashes between Joselewicz's squadron with Austrian Hussars stationed in Kock, the Austrian Hussars were forced to retreat, but Joselewicz was killed during conflict. Historical sources surrounding the circumstances of his death are unclear; an article published by Ernest Łuniński in Tygodnik Illustrowany in 1909, 100-years after his death, suggests he may have been trampled by horses, but the article does not provide a source for this information.

Berek's son, Josel Berkowicz, also fought in the Battle of Kock in 1809, which occurred around the time he reportedly entered military service. He later served as a squadron chief during the November Uprising in 1830. He and his mother, Rokhla, reportedly received military pensions from the Polish Government until 1831.

In his youth, he received a traditional Jewish education. In 1809, at 20 years old, he joined the Army of the Duchy of Warsaw and took part in the war between the Duchy of Warsaw and Austria. He fought at his father's side during the Battle of Kock, and also took part in the war with Russia in 1812 with the rank of lieutenant. For his military achievements, he was awarded the Legion of Honor with the Golden Cross and other decorations. He requested to be discharged from service around 1815 due to injuries received during the war, and subsequently worked as a forest warden in several areas of Poland for a few years.

In 1830, he took part in the November Uprising against Russian control of Congress Poland. Following the example set by his own father, he approached the leadership of the uprising under General Józef Chłopicki with a proposal to create a battalion of Jewish volunteers in support of the rebellion and subsequently issued a public appeal:

He issued several public appeals to Polish Jews to join his battalion, but was less successful than his father due to resistance from the Warsaw kehilla leaders; the rabbi of Kock at the time, Menachem Mendel Morgensztern​, was supportive of the rebellion.

The uprising was ultimately unsuccessful, and his involvement resulted in exile. He reportedly lived in France from 1832 to 1836, when he moved to England. While living in England, he reportedly wrote a fictional account of his father’s story, which his sons published after his death in 1846 under the title Stanislaus, or the Polish Lancer in the Suite of Napoleon, from the Island of Elba​. He died in England in 1846.