Chadwell-St-Mary is in the unitary authority of Thurrock in Essex, England. It is one of the traditional (Church of England) parishes in Thurrock and a former civil parish. Its residential areas are on the higher ground overlooking the former Thameside marshland. Grays is a town centred west of it. Closer still, south is the modern town of Tilbury which was almost wholly part of the parish until the end of the 19th century. The settlement is frequently referred to simply as Chadwell, particularly before the 19th century.