Prehistoric and Roman finds have been recovered from the old land surface below the castle earthworks. There is a Roman villa to the west of the town at Plesheybury.
Pleshey originated as a motte and bailey castle, built in the mid-twelfth century by Geoffrey de Mandeville on the edges of High Easter parish. The northern bailey is now marked by the curving line of Back Lane. In the late-twelfth century there was a second phase of building, probably as a result of the granting of permission to refortify the castle in 1167 following its partial demolition in 1157 as a consequence of Geoffrey de Mandeville IIís arraignment for treason. This building phase consisted of the construction of the present southern bailey and probably the town enclosure. Permission was also granted to build a chapel to serve the new town. The town enclosure is semi-circular in plan, enclosing an area of c.14 ha., the castle forming the southern limit of the town.
It is not clear whether the medieval town ever filled its enclosure, but the post-medieval town certainly did not. The street pattern consists of the east-west main street, a back lane which follows the line of the original bailey defense and a road running from the back lane to the north gate. The original chapel and St Maryís Church stood in the north-west quadrant behind the back lane. However, in 1393 a college of canons and a new church were founded by the Duke of Gloucester, the then owner of the castle just outside the town enclosure. The south-west corner of the bailey was altered as a result to form a right-angled corner and a moated enclosure for the college. During the medieval period it was first the seat of the High Constables of England (the de Mandevilles and the de Bohuns) and then part of the marriage-portion of the Queens of England.
Following the abandonment of the castle in the mid-sixteenth century Pleshey declined to village status and has grown little since that date. In the nineteenth century over 70% of the inhabitants of Pleshey were agricultural labourers employed locally.