This map shows the parish boundaries as held by the Church Commissioners:
A church of St Mary stood until the early 16th Century on the old site of the public library at Old Cross. St Nicholas Church stood in Maidenhead Yard (behind Maidenhead Street). During the 16th Century the population decreased and both churches fell into disuse; they were closed and the parishes annexed to St Andrew's early in the 18th Century.
From the Castle westward, the parish comprises the centre of the town bounded by the river Beane to the north and the rivers Lea and Mimram to the south. Beyond the housing estates of Sele Farm and Hertingfordbury Road it is bounded to the west by the rural parishes of Waterford, Tewin, Bramfield and Hertingfordbury. The total population of the parish is around 7500.