[1] Purton is about 4 miles (6 km) north-west of the large town of Swindon, and a similar distance south of Cricklade.

Nikolaus Pevsner describes the church as "beautifully placed";[2] it stands separate from and to the south of the present-day village, next to the 16th-century manor house.

[4] The quality of the construction and interior adornment of the church may reflect the wealth of Malmesbury Abbey.

[7] The west tower, called "ambitious" by Pevsner,[2] is 15th-century; the upper stage and slender octagonal spire over the crossing may be 14th-century[1] or 15th.

[7] The church is constructed of coursed rubble limestone, with roofs of slate and lead.

[1] St Mary's is one of only three churches in England to have both a western tower and a central spire.

The first and second bells (the lightest) were cast in 1989, and the seventh in 1916 by John Taylor & Co of Loughborough; the third and fifth by Gillett & Johnston of Croydon in 1924 and 1923 respectively; the fourth by Robert Wells II in 1793; the sixth by Joseph Carter in 1598, the largest bell known to have been cast by this founder; and the tenor (heaviest) by Abel Rudhall of Gloucester in 1738.

[1] While the exterior of the building is largely Perpendicular in style,[4] Pevsner writes that the church is "very different inside from what the outside indicates".

[12] The surviving parish registers date from 1558 for marriages and burials, 1564 for baptisms;[13] there are gaps between 1641 and 1647 which coincide more or less with the disruptions of the Civil War.