During a continental tour in 1825 and 1826, in which his companions were Mark Napier, Hamilton Gray, and Sir Charles Fergusson, the art and literature of Italy first engaged Dennistoun's attention.
In 1833 "James Dennistoun, advocate" is listed as living at 1 Albyn Place, a huge house at the north-east of Edinburgh's First New Town on the edge of the Moray Estate.
He formed a collection of early Italian pictures, drawings, and mediæval antiquities, with which he adorned his house in George Street, Edinburgh, his permanent home from 1847.
For the Maitland Club, Cartularium comitatus de Levenax, ab initio seculi decimi tertii usque ad annum MCCCXCVIII., 1833; the Cochrane Correspondence regarding the Affairs of Glasgow 1745–6, 1836; the Coltness Collections 1608–1840, 1842, and, as co-editor with Alexander Macdonald, Miscellany, consisting of Original Papers illustrative of the History and Literature of Scotland, vols.
and iii., 1834, &c. Dennistoun also wrote a Letter on the Scotish [sic] Reform Bill by a Conservative, 1832; Memoirs of Sir Robert Strange, engraver, and of his brother-in-law, Andrew Lumisden, private secretary to the Stuart Princes, 1855, 2 vols.