Isaac Rosenberg was born in Bristol, the second of six children and eldest son of his parents, Barnett (formerly Dovber) and Hacha Rosenberg, who were Orthodox Jews from Dvinsk (now in Latvia). In 1897, the family moved to 47 Cable Street in a poor district of the East End of London, and one with a strong Jewish community. He attended St. Paul's School Whitechapel around the corner in Wellclose Square, until his family (of Russian descent) moved to Stepney in 1900, so he could experience Jewish schooling, and he attended the Baker Street school. He left school at the age of fourteen and became an apprentice at a firm of engravers in Fleet Street.
He was interested in both poetry and visual art, and attended evening classes at the Birkbeck College art school. He completed his apprenticeship in 1911, and managed to find the finances to attend the Slade School of Fine Art at University College, London (UCL). During his time at Slade School, Rosenberg notably studied alongside David Bomberg, Mark Gertler, Stanley Spencer, Paul Nash, Edward Wadsworth, Dora Carrington, William Roberts, and Christopher Nevinson. He was taken up by Laurence Binyon and Edward Marsh, and began to write poetry seriously, but he suffered from ill-health.
He published a pamphlet of ten poems, Night and Day, in 1912. He also exhibited paintings at the Whitechapel Gallery in 1914.
Afraid that his chronic bronchitis would worsen, Rosenberg hoped to try to cure himself by emigrating in 1914 to the warmer climate of South Africa, where his sister Mina lived in Cape Town.
He wrote the poem "On Receiving News of the War" in Cape Town. While others wrote about war as patriotic sacrifice, Rosenberg was critical of the war from its onset. However, needing employment in order to help support his mother, Rosenberg returned to England in October 1915, where he published a second pamphlet of poems, Youth and then enlisted in the British Army.