st mary's is noted for its interior which was renovated in 1850 by the architect george gilbert scott and includes fine oak pews decorated in the gothic revival style. it also contains the essex chapel, which served at the burial place of the nobility of the cassiobury estate, including the earls of essex. the chapel contains a number of large, ornate marble tombs and memorials dating from the 16th century and later, and was described by pevsner as "the chief glory of watford church".