St Mary's houses a complete set of 28 pre-reformation stained glass windows. The medieval  panes are of national historical and architectural importance as they constitute what is "probably the most complete set of medieval stained glass in Britain" consisting of 28 windows displaying biblical scenes. They were added after the church had been rebuilt by the wealthy wool merchant John Tame (c.1430–1500). The glass was made between 1500 and 1517 under the instructions of his son, Edmund Tame.

The windows are of enormous value to us because they are a complete scheme. It is clear there was a theological plan of what was to be illustrated and what position it should occupy. You can enjoy yourself on a visit trying to work out why a particular episode or character has been included and what their position might tell you. For example why is the prophet David facing the apostle James the Great?

The windows had not been in long before the religious upheavals of the 16th century threatened them. Clear orders were issued in Edward VI’s reign to rid churches of all idolatrous images. In parts of the country this led to great destruction of glass and another period of similar glass breaking happened in the early 17th century. We don’t know how Fairford’s glass survived but there are theories e.g. whitewash, protection by influential people.

Ironically it was an “act of God” that caused the greatest destruction to the windows. In November 1703 a great storm across southern England caused massive damage and loss of life. The three west-facing windows were damaged and the vicar came in the following morning to find glass strewn across the floor. The windows were repaired as best they could but there were gaps and mistakes. In the middle of the 19th century there was a move to repair the Great West Window and a company called Chance Brothers of Smethwick took away the top half. Instead of repairing it they replaced it. Unease about this and the replacement of two of the apostles led to a halt to anything but careful mending of leads and limited restoration.

In 1986 Keith Barley was commissioned to begin a full conservation and restoration programme which began in 1988 and finished in 2010. The windows should now be good for a century of two. The ancient glass is no longer exposed to the attrition of the weather – clear glass protects the vulnerable old glass while inside condensation is avoided by allowing air to circulate both sides of the glass. We have almost no record of how in earlier times Fairford’s glass managed to survive but we do know the townspeople today are determined to safeguard its future.