The best contemporary account of his life appeared in the Gentleman's Magazine for June, 1855, which he was so fond of quoting.
"Mr. Dennistoun," we read there, "was born in Dumbartonshire in 1803, and was the representative of the knightly house of Danzielstoun in Renfrewshire, one of the oldest Scottish families. He was educated at the College of Glasgow and qualified himself for the Bar in Edinburgh; but his taste took a different direction, and being possessed of sufficient fortune, he turned aside from the legal profession and devoted his whole attention to literature, in connection chiefly with the Fine Arts. He was an amateur of Art according to the true and proper meaning of that designation—he loved and admired Art, and studied to appreciate the best examples that the world possesses. Though in following out these studies he devoted much of his time to the Italian school, as there painting first arose in strength, yet he was no bigoted admirer, and could appreciate the qualities of all kinds of Art, whether Italian or German, ancient or modern. He then aimed at giving to the public the ideas he had formed regarding its principles, and the facts he had collected as to its history. He could not unfold before all his friends and visitors portfolios filled with sketches, done by himself, of passes in the Alps, or of scenery in the Tyrol, or of views of the Temple of the Sibyl at Tivoli, of Mount Vesuvius, etc.; but to all who wished to learn, he could impart in a manner the most simple and unpretending, but with a clearness and elegance that impressed and charmed all who were privileged to hear him (and these were many), information and instruction on almost everything relating to Art: while he often explained and illustrated what he stated by reference to examples he had himself collected—many of them of great rarity and value.
"He was a member of most of those societies formed for collecting materials for, and adding to and illustrating the literature of Scotland, and besides editing several important publications by the Bannatyne and Maitland Clubs, contributed many interesting papers on subjects connected with Art to most of the leading periodicals, particularly to the Edinburgh and Quarterly Reviews.
"His first work, we believe, was the edition of Moysie's Memoirs of the Affairs of Scotland from 1577 to 1603, which he contributed to the Bannatyne and Maitland Clubs in 1830. This was followed by the Cartularium Comitatus de Levenax, ab initio seculi decimi tertii usque ad annum MCCCXCVIII, edited by Mr. Dennistoun, and printed for the Maitland Club by Mr. Campbell of Barnhill. In 1834 another illustration of Lennox history proceeded from Mr. Dennistoun's pen, in a reprint of The Lochlomond Expedition, with some Short Reflections on the Perth Manifesto, 1715. He also edited the volume of The Coltness Collections, 1608-1840, for the Maitland Club, in 1842. The Ranking of the Nobility, 1606, was printed, along with some other papers, in The Miscellany of the Maitland Club.
"A residence in Italy gave a new bent to his pursuits. One of the first-fruits of these Transalpine studies was a deeply interesting paper on The Stuarts in Italy, published in the Quarterly Review for December, 1846. But by far the most considerable result of Mr. Dennistoun's Italian sojourn was his Memoirs of the Dukes of Urbino, published in three volumes in 1852. This work is of great value, as illustrating the state of Italy during the fifteenth and sixteenth centuries, the portion devoted to the Arts of the period being particularly interesting; and it is to be regretted that from a delicacy carried perhaps too far, he has curtailed this important section—the one he could best handle—from fear, as he states in the preface, of trenching on ground entered on by his friend, Lord Lindsay.
"Mr. Dennistoun was the writer of the article on Mr. Barton's 'History of Scotland' in the Edinburgh Review for October, 1854; and also of the analysis lately given in the same periodical of the Report by the Commission on the National Gallery, which is very masterly, and, indeed, the only successful attempt yet made to grapple with that huge accumulation of facts and opinions of all kinds.
"He had just lived to complete another very interesting work, consisting of the Memoirs of Sir Robert Strange, the excellent engraver, and of his brother-in-law, Andrew Lumisden, secretary to the Stuart princes, and author of the Antiquities of Rome. Sir Robert Strange was the maternal grandfather of Mrs. Dennistoun. To that lady, Isabella-Katharina, eldest daughter of the Hon. James Wolfe Murray, Lord Cringletie, a Lord of Session, Mr. Dennistoun was married in 1835."
In the Report from the Select Committee on the National Gallery, published by order of the House of Commons in December, 1853, we find Dennistoun as one of the witnesses. His evidence appears to have been of some value, and the articles which he wrote for the Edinburgh Review, both before and after the Report was published, are excellent both in tone and substance.
"You are the possessor," he was asked, "of a small and, I may say, very choice collection of Italian pictures, are you not?"
"A collection of early Italian pictures," he answered. And, indeed, in his day such a collection must have been very rare in England, or, in fact, anywhere else. These pictures were sold with other works of art that had been in his possession, on Thursday, June 14, 1855, and by the courtesy of Messrs. Christie, Manson & Woods, of King Street, St. James's, I am able to print the [catalogue] they prepared for the sale, and the prices the pictures fetched.