With a regular income, Hahn was now able to contemplate marriage. In June 1911, while attending a conference in Stettin, Hahn met  (1887–1968), a student at the Royal School of Art in Berlin. They saw each other again in Berlin, and became engaged in November 1912. On 22 March 1913 the couple married in Edith's native city of Stettin, where her father, Paul Ferdinand Junghans, was a high-ranking law officer and President of the City Parliament until his death in 1915. After a honeymoon at Punta San Vigilio on Lake Garda in Italy, they visited Vienna, and then Budapest, where they stayed with George de Hevesy.
Their only child, , was born on 9 April 1922. During World War II, he enlisted in the army in 1942, and served with distinction on the Eastern Front as a panzer commander. He lost an arm in combat. After the war he became a distinguished art historian and architectural researcher (at the Hertziana in Rome), known for his discoveries in the early Cistercian architecture of the 12th century. In August 1960, while on a study trip in France, Hanno died in a car accident, together with his wife and assistant Ilse Hahn née Pletz. They left a fourteen-year-old son, Dietrich Hahn.
In 1990, the  for outstanding contributions to Italian art history was established in memory of Hanno and Ilse Hahn to support young and talented art historians. It is awarded biennially by the Bibliotheca Hertziana – Max Planck Institute for Art History in Rome.