St Mary’s Old Amersham dates from around 1140 A.D. though the site has been a holy one for much longer, as it stands where Roman Road crosses the little River Misbourne. The missionary monks of St Augustine, St Birinus and indeed earlier evangelist Bishops would baptise their converts at just such a location.

As the town expanded during the middle ages, the church was extended, widened and heightened, much work being done in the C14 and C15, when the building came to look much as it does today.

[Excerpt from Chiltern Arts website]