Witt read history at New College, Oxford and qualified as a solicitor in 1897. In 1899 he married Mary Helene Marten, a fellow Oxford student who, like him, collected photographs and reproductions of works of art. Their joint collection, eventually surpassing 500,000 items, was housed in their home at 32 Portman Square, London. The Witt Library, as the couple referred to it, was the world's largest archive of reproductions of paintings and drawings, and turned their home into an international study center for scholars of art history. The collection was bequeathed to the Courtauld Institute upon the couple's death in 1952.