At the instance of Dr. Hooper of Pant-y-Goetre and Morgan of Tredegar, Williams undertook to write a history of Monmouthshire, and in 1792 visited the county to collect materials. Shortly afterwards Roland, during his second term of office as minister of the interior, invited Williams to Paris. He went over about August 1792, was made a French citizen, and remained till the execution (21 January 1793) of Louis XVI, a measure which he strongly deprecated. While in Paris he published "Observations sur la derniere Constitution de la France", 1793. He brought with him, on his return, a letter to William Grenville, 1st Baron Grenville, from Lebrun, minister of war, who wished to make Williams a medium of communication between the two governments; but no notice was taken of it. An engagement previously entered into for completing the continuation of Hume's "History of England" was cancelled, owing to the political odium incurred by his visit to France. His "History of Monmouthshire", 1796, with illustrations drawn and partly engraved by his friend Gardner, and a very modest introduction, was considered a standard work on the subject; unfortunately it has no index.