It is an active church situated in the town centre on Watford High Street, approximately 25 kilometres (16 mi) outside London.

Thought to be at least 800 years old, the church contains burials of a number of local nobility and some noteworthy monumental sculpture of the Elizabethan and Jacobean eras.

During renovations in 1871, church restorers discovered that 12th-century stonework had been incorporated into the later medieval building, and in the hardcore of the tower walls, the basin of a 12th-century baptismal font had been discarded.

[5] Constructed in stone and faced in flint, the church has a broad clock tower at the west end typical for Hertfordshire, topped with crenelations.

Seen from the outside, St Mary's is for the most part a 15th-century building; the tower, outer walls of the aisles, clerstory, nave roof and south chancel chapel all date from this period.

By the time the church was being recorded by the historian John Edwin Cussans in his History of Hertfordshire (1880), the Heydon Chapel was in use as an organ chamber.

Of particular note is a white marble tablet in the Choristers' Vestry to the memory of Robert Clutterbuck, the author of a History of Hertfordshire.

[7] The oak pews installed at this time were especially noted as a fine example of carved ornamentation in the Decorated Gothic style with tracery heads and foliate spandrels.

[11] A refurbishment scheme for the church interior was announced in 2014, which involved laying new flooring, installing internal plate glass screens, and the removal of George Gilbert Scott's oak pews to replace them with modern upholstered chairs.

There are many other minor monuments, including a marble memorial to Anne Derne (d.1790), daughter of John Carpender, by J. Golde of High Holborn.

[15][16] The most striking memorials in the chapel are two large wall monuments executed by the sculptor Nicholas Stone, described by Pevsner as "the chief glory of Watford Church".

Morrison is surrounded by an ornate canopy of twin segmental arches supported by two pillars of coloured marble, the family coat of arms, and at either end figures of his son and daughter kneel under sculpted fabric baldacchinos.

After their death in the early 1800s, a fig tree did indeed grow out of this tomb for many years, breaking through the stone until it was killed by the severe winter of 1962/63.