John Parker had considerable alterations and restoration work carried out to his own designs and largely at his own expense to the churches at Llanmerewig and Llanyblodwel. To the former he added a tower and a porch, and to Llanyblodwel a tower; he also built a school and a schoolmaster's house in Llanyblodwel. He d. 31 Aug. 1860, and was buried in Llanyblodwel churchyard. He had succeeded to the Sweeney Hall estate in 1854 on the death of his father (who was a designer of houses and a writer of books), but as he himself died unmarried, the estate passed to his sister, Mary Parker, lady Leighton (see below).

Parker will be remembered as a remarkably prolific amateur artist, chiefly in water-colour. His principal interests appear to have been scenic effects and Gothic architecture. He visited the Snowdon district ten years in succession; he made drawings of all the rood screens in Welsh churches and of all the chief types of fonts. Over a thousand drawings by him are kept in the National Library of Wales. Besides the Welsh, English, and Irish views and sketches, there are drawings of details from churches in Europe; he was also a competent flower painter. He had published, in 1831, The Passengers: containing the Celtic Annals.