Jenny Marx Longuet, called "Jennychen" to distinguish her from her mother, was born in Paris, France, the eldest child of Karl Marx and his wife Jenny von Westphalen. When she was a small child, the family moved frequently due to her father's political writing, finally settling in London. Jenny became a French and German language teacher and wrote articles for the socialist press under the pen name J. Williams. In 1872, she married Charles Longuet, a French journalist and radical political activist in exile, with whom she would have six children. The couple moved to Oxford, and Jenny taught at the Clement Dane School and gave private lessons to supplement their meagre income. Her husband later got a teaching position at King's College, London, and they moved back there. In 1880, the French government offered a political amnesty, and Charles Longuet quickly took advantage of it to return to his native country. Jenny and her three sons stayed behind, as
she was looking after her ailing parents, and was also ill herself. In 1881, she and the children joined her husband at a home in Argenteuil. She died there at age 38, probably from cancer.
A collection of letters, The Daughters of Karl Marx: Family Correspondence, 1866-1898, published in 1982, lists Jenny Marx Longuet as the first author.