Hildegard Knies trained as a social worker in 1937 and began studying psychology in Berlin in 1940. She took a critical approach to National Socialism, supported by her former teachers Elisabeth Abegg and Elisabeth Schmitz. Along with Abegg, Hildegard Knies was active in the Social Working Group Berlin-East (SAG). This community provided social work in poorer parts of Berlin.
From 1942, Hildegard Knies supported Elisabeth Abegg in helping persecuted Jews. Knies hid several Jews in her small Charlottenburg apartment, including Liselotte Pereles.
After the war, Hildegard Knies married Kurt Arnold. She joined the SPD and worked in family advice and neighborhood support services in West Berlin for decades.
For her rescues of persecuted Jews during National Socialism, Hildegard Arnold received the Federal Cross of Merit in 1986. In 2007, ten years after her death, the Israeli Holocaust memorial center Yad Vashem honored her as Righteous Among the Nations.