Born in Hamburg, she was the eldest daughter of poet Rosa Maria Varnhagen, raised as a Lutheran, and David Assur, a Jewish physician, who converted to Christianity upon marriage and changed his name to Assing.

[1] Her mother was friendly with other literary women, including Clara Mundt and Fanny Lewald, and prominent in liberal circles that supported (but failed to achieve) social revolution in 1848.

[2] After the deaths of their parents and the Great Fire of Hamburg in 1842, Assing and her sister Ludmilla went to live with their uncle Karl August Varnhagen von Ense, a prominent literary figure and revolutionary activist.

Through the hundreds of articles she wrote, hers became one of the central voices in interpreting abolitionism and the realities of the United States' slave-holding society for European audiences.

[5] She also gave him shelter when "he was on the run from conspiracy charges in connection with John Brown's raid on Harpers Ferry", when he was at risk for capture and execution.

"[6] In 1884, having already been diagnosed with incurable breast cancer, Assing was in Europe trying to establish her claim to her sister's estate when she learned that Douglass had married Helen Pitts, a younger white woman who worked with him as his secretary in the Recorder's Office.