The following materials can be found in James Donald Milner's papers:
- Papers concerning his work for the National Portrait Gallery, comprised of correspondence concerning general gallery business (1893-1895) and a report on portraits belonging to the National Portrait Gallery temporarily on display or in storage at the National Gallery (1894)
- Portrait research comprising a series of small sketchbooks (1893-1903) with hand drawn copies of portraits, in pencil, with descriptive notes and a notebook regarding portraits in the Dilettanti Society collection at the Grafton Gallery (1896)
- Correspondence and research notes (1925-1927) regarding Tilly Kettle
- Private papers (1909-1918) comprising one small personal notebook; letter from his brother Archibald Donald Milner written whilst serving in France during the First World War and a file of correspondence to Milner from various senders, largely regarding his appointment as Director in 1916 (also three warrants and two envelopes, financial records of his brother Archibald (1911-1914))