St Mary's is in the patronage of the Duchy of Cornwall. The orginal white ensign was presented to St Marys in 1905 by the then Duke of Cornwall, (the future George V) on the occasion of the marriage of Lionel Halsey, an officer in the Royal Navy, to Morwenna Granville, the daughter of Bevel Granville, the owner of Northchurch Hall.
Fifteen years later this flag was used to cover the newly constructed Northchurch War Memorial which was unveiled on behalf of the new Duke of Cornwall, (the future Edward VIII), by Rear Admiral Lionel Halsey.
Following the unveiling of the War Memorial on 6th March 1920 a replacement flag was given to St Mary's by Edward, Duke of Cornwall.
The current flag was donated to the church by the current Duke of Cornwall, Prince Charles, in 1971.