de Ropp was born in Bath, England, on February 1, 1913, the son of William de Ropp (originally Wilhelm von der Ropp) by his marriage to Ruth Fisher. The Ropp family had been land-owning barons in Lithuania. William was of Teutonic and Cossack descent, being entitled to use the title “Baron”, and was perpetually in shaky financial circumstances. He had settled in England in 1910 and become naturalised in 1913. de Ropp's mother, Ruth, was a daughter of Albert Bulteel Fisher, whose brother was the academic historian Herbert William Fisher. Ruth de Ropp died in the 1918 flu pandemic. Robert de Ropp had also contracted the flu during the pandemic, and by the time he fully recovered from its ravages he was seven years old.