Claire was born in Belin, Germany. Her parents were Arthur and Jeanette (née Jacobi) Lewisohn. She married Günther Emmanuel Loewenfeld (November 1895–January 1984) on 5 July 1921. They continued to live in Berlin in the period followng their marriage. Both Claire and Günther were from Jewish families, however, Günther was brought up in the Protestant faith. Between 1923 and 1925 they spent their weekends with friends Fritz and Lily Pincus in a rented house, in Glienicke, on the outskirts of Potsdam. In 1925 the Loewenfelds and Pincuses moved out of Berlin to a rented property which they shared on the Küssel, a peninsula jutting out into Lake Templiner in a rural district of Potsdam. Both husbands commuted to Berlin to work. By 1931 Claire and Günther had two children, Peter and Verena, likewise the Pincuses had two children. Both couples also had their relatives living with them from time to time and as more living space was needed they decided to jointly buy the property and enlarge it.
Das Haus auf dem Küssel (The House on the Küssel) as it had become known was redesigned, to include both shared areas and private quarters, by a well-known Potsdam architect, Stephan Hirtzel. Another close friend of both families, Paul Tillich, a German-American Protestant theologian wrote a dedication on the inauguration of their new home entitled, (in English),
Space and Time in Dwelling. The Loewenfeld and Pincuses' house soon became a meeting place for Tillich and his circle of German intellectuals until Tillich, whose writings brought him into conflict with the Nazi movement, was subsequently forced into exile in the U.S.
During early 1936 the Loewenfelds travelled to Syria and Palestine where they witnessed at first-hand the initial stages of the Arab uprising against British mandate and Jewish immigration. They spent the summer of 1936 near Cortina in the Italian Dolomites where they met Tillich who was on a European lecture tour. In Tillich's diary an account of their time in Palestine records:
"While in Palestine, Claire and Guenther were in constant danger of their lives. Once, the only thing that saved them was their Arab guide saying they were German Nazis. Hitler is the big man with the Arabs. Mussolini gives them money to spite the British."
From 1937 Claire and Lily's home in the Küssel provided a refuge for Jewish children, whose parents had been arrested or had been abandoned and were homeless. Claire's family continued to live in Germany until the latter part of 1938 when they left Potsdam due to the increasing likelihood of arrest. The Loewenfelds had made arrangements in advance for their belongings to be transported to England and for their children to be evacuated to an English boarding school, St Christopher School, Letchworth, Hertfordshire. Meanwhile Günther joined relatives in England and Claire travelled first to Switzerland before rejoining her husband in early 1939. The family settled in rural Buckinghamshire in 1941.