The parish of Monken Hadley was part of Middlesex until 1889. There are several places of significance in Hadley. The village area itself was called Monken Hadley, centred on the Anglican church of St Mary. East of this was Hadley Common. West of the village is Hadley Green, north of which is Hadley Highstone.
Hadley was named in a description of the boundary of the manor of Barnet in 1005. Hadley means a heath-like clearing in a wood. Hadley does not appear in the Domesday survey. In 1136 Geoffrey de Mandville gave the monks at Walden Abbey in Essex part of his land.
It is this monastic association that gives the Monken part of the name, which was in use by the 1170s. There was a small hermitage, a retreat for the monks, and a parish church. Many authorities believe that Hadley Green was location of the Battle of Barnet in 1471.