W tower, four-bay aisled nave, chancel aisled on the S side only. The S nave arcade dates from the late 12thc. The tower is largely 16thc., having been rebuilt after a storm in 1593/94, and contains 12thc. moulded stones taken from the abbey. The history of the N aisle is a complex one. There was a N transept in the 14thc., in which the Jesus Chantry was housed, but the N nave aisle which it now terminates was not added until 1872. In 1918 a small chapel, the St Edward's or War Memorial Chapel, was built out from the N wall of the aisle. The chancel was extended and given its S aisle in 1863. This S chancel aisle was originally a Lady Chapel, but is now a candle shop. The only 12thc. sculpture is on the capitals of the S nave arcade.

The early history of the foundation is extremely speculative. The name of Minster Street, which marks the southern boundary of the churchyard, dates back at least to the 13thc. and suggests that the church once enjoyed minster status. It has been assumed to be the site of the abbey referred to in the Reading Abbey Foundation charter as having been destroyed on account of its sins, and its revenues granted to the new foundation. It is further suggested that it was founded by Elfrida, second wife of King Edgar, as a nunnery in 979. The only Reading church recorded in DS was held by the Abbot of Battle, and previously belonged to Abbess Leofeva of Shaftesbury. It has been suggested that Shaftesbury, which housed the body of Edward the Martyr, reputedly murdered by Elfrida, received Elfrida's Reading nunnery as an act of expiation. The present St Mary's can only be traced to 1173 when its first vicar, William de Lincoln, was appointed. It was substantially rebuilt between 1551 and 1555 using timber, stone, lead and glass from the dissolved abbey. The steeple was blown down in 1593 or '94 and repairs continued until 1598. It is not clear whether the reused abbey fabric in the tower was inserted in the 1551-55 or the 1594-98 campaign.

The churchwardens' accounts of 1550-55 record that 20 shillings were paid to George Hynde, who was responsible for disposing of the abbey fabric after the Dissolution, for "the pyllers (out of the Abbye)." It has been sometimes assumed that these pillars are those of the south nave arcade, and that the south aisle therefore dates, in its present form, from the 1550s rather than circa 1200. The present author is not persuaded that these were the pillars purchased from the abbey. They are much too insubstantial to come from the abbey church, which was the source of most of St Mary's purchases, and their style of carving indicates a date which corresponds to no known building campaign at the abbey. It seems likelier that columns from the choir or the nave of the abbey church were broken up for use as building material during the major rebuilding of St Mary's of 1551-55.