As a shire Hertfordshire is of purely military origin, being the district assigned to the fortress which Edward the Elder erected at Hertford. It is first mentioned in the Saxon Chronicle in 1011. At the time of the Domesday Survey the boundaries were approximately those of the present day, but part of Meppershall in Bedfordshire formed a detached portion of the shire and is still assessed for land and income tax in Hertfordshire. Of the nine Domesday hundreds, those of Danais and Tring were consolidated about 1200 under the name of Dacorum; the modern hundred of Cashio, from being held by the abbots of St Albans, was known as Albaneston, while the remaining six hundreds correspond approximately both in name and extent with those of the present day.
Hertfordshire was originally divided between the dioceses of London and Lincoln. In 1291 that part included in the Lincoln diocese formed part of the archdeaconry of Huntingdom and comprised the deaneries of Berkhampstead, Hitchin, Hertford and Baldock, and the archdeaconry and deanery of St Albans; while that part within the London diocese formed the deanery of Braughing within the archdeaconry of Middlesex. In 1535 the jurisdiction of St Albans had been transferred to the London diocese, the division being otherwise unchanged. In 1846 the whole county was placed within the diocese of Rochester and archdeaconry of St Albans, and in the next year the deaneries of Welwyn, Bennington, Buntingford, Bishop Stortford and Ware were created, and that of Braughing abolished. In 1864 the archdeaconries of Rochester and St Albans were united under the name of the archdeaconry of Rochester and St Albans. In 1878 the county was placed in the newly created diocese of St Albans, and formed the archdeaconry of St Albans, the deaneries being unchanged.
Hertfordshire was closely associated with Essex from the time of its first settlement, and the counties paid a joint fee-farm and were united under one sheriff until 1565, the shire-court being held at Hertford. The hundred of St Albans was at an early date constituted a separate liberty, with independent courts and coroners under the control of the abbot; it preserved a separate commission of the peace until 1874, when by act of parliament the county was arranged in two divisions, the eastern division being named Hertford, and the western the liberty of St Albans. These divisions have since been abolished.