The only information available to us for John Tonkin comes from an advertisement in the Royal Cornwall Gazette, 5 March 1808. This tells us Tonkin was a miniature and landscape painter in Truro. He was also recorded by Foskett as working in Penzance in 1830.
Source: McKechnie (Author of, British Silhouette Artists and their Work 1760-1860)
Tonkin, John (McKechnie Section 2)