 He went with Thomas Edlyne Tomlins to cathedrals in England and Ireland to search for original statutes; and he also inspected the state of the records in Ireland.In about 1805 he was chosen deputy-keeper of the records in the Tower of London under Samuel Lysons.When Henry Petrie succeeded Lysons as keeper in August 1819, he refused to continue Illingworth's employment as deputy-keeper, offering him instead a post as a clerk