At the Westminster he had a reputation as a reformer, and for palliation and cures of cancers. Justamond also acted as surgeon to the 2nd Regiment of the Dragoon Guards. He was a Fellow of the Royal Society.
Justamond was also employed by the British Museum as a deputy keeper, a locum for Daniel Solander.
Shortly after Maty died he fell into debt, and lost his museum position of Assistant Librarian in 1778, being replaced by Edward Whitaker Gray.