He was born Boruch-Aharon Thomashefsky in Osytnyazhka [uk][2][3] (Ukrainian: Оситняжка; Yiddish: אָסיטניאַשקע), a village in the Chyhyryn county of the Kiev Governorate of the Russian Empire (today in the Kirovohrad Oblast, Ukraine).

Thomashefsky, who was earning some money by singing on Saturdays at the Henry Street Synagogue on the Lower East Side, was also working as a cigarette maker in a sweatshop, where he first heard songs from the Yiddish theater, sung by some of his fellow workers.

The performance was a mild disaster: pious and prosperous "uptown" German Jews opposed to the Yiddish theater did a great deal to sabotage it.

After Yiddish theater was banned in Russia, his tours came to include such prominent actors as Siegmund Mogulesko, David Kessler, and Jacob Adler, with new plays by playwrights such as Moses Ha-Levi Horowitz.

[5] In 1887, playing in Baltimore, he met 14-year-old Bessie Baumfeld-Kaufman, when she came backstage to meet the beautiful young "actress" she had seen on stage, only to discover that "she" was Boris.

Bessie soon ran away from home to join the company, and eventually took over the ingenue roles, as Boris moved on to romantic male leads.

[6] After Adler recruited Jacob Gordin as a playwright and found a way to draw the masses to serious theater with Gordin's The Yiddish King Lear, and then turned to Shakespeare's Othello, Thomashefsky decided to show that he could compete on that ground as well, and responded with the first Yiddish production of Shakespeare's Hamlet, in which, by all reports, he acquitted himself excellently.

Claudius spreads a rumor that Prince Hamlet has succumbed to nihilism while away, but his scheme is discovered and the traitor is sent to Siberia in his nephew's stead.

[8] Other notable Thomashefsky productions included Yiddish versions of Uncle Tom's Cabin, Goethe's Faust and, unlikely as it may seem, Wagner's Parsifal.

The first son, Harry, went on to direct the film The Yiddish King Lear (1935), under the auspices of the Federal Theatre Project,[14][15] and later moved with his mother to Los Angeles.

[18] Thomashefsky is buried with his wife, who, although separated from him by 1911, never divorced him, in the Yiddish theater section of the Mount Hebron Cemetery, Flushing, Queens, New York.