It is one of the home counties and is bordered by the non-metropolitan counties of Bedfordshire to the north, Cambridgeshire to the north-east, Essex to the east, and Buckinghamshire to the west, while Greater London lies to the south. The county town is Hertford.

Hertfordshire was originally the area assigned to a fortress constructed at Hertford under the rule of Edward the Elder in 913. Hertford is derived from the Anglo-Saxon heort ford, meaning deer crossing (of a watercourse). The name Hertfordshire is first recorded in the Anglo-Saxon Chronicle in 1011.

Deer feature in many county emblems.
There is evidence of humans living in Hertfordshire from the Middle Stone Age. It was first farmed during the Neolithic period and permanent habitation appeared at the beginning of the Bronze Age. This was followed by tribes settling in the area during the Iron Age.

Following the Roman conquest of Britain in AD 43, the aboriginal Catuvellauni quickly submitted and adapted to the Roman life; resulting in the development of several new towns, including Verulamium (St Albans). It was here in c. 293 that the first British martyrdom took place. St. Alban, an Anglo-Roman soldier, took the place of a Christian priest and was beheaded on Holywell Hill. His martyr's cross of a yellow saltire on a blue background is reflected in the flag and coat of arms of Hertfordshire as the yellow background to the stag or Hart representing the county. He is the Patron Saint of Hertfordshire.

With the departure of the Roman Legions in the early 5th century, the now unprotected territory was invaded and colonised by the Anglo-Saxons. By the 6th century the majority of the modern county was part of the East Saxon kingdom. This relatively short lived kingdom collapsed in the 9th century, ceding the territory of Hertfordshire to the control of the West Anglians of Mercia. The region finally became an English shire in the 10th century, on the merger of the West Saxon and Mercian kingdoms.

A century later the victorious William of Normandy received the surrender of the surviving senior English Lords and Clergy, at Berkhamsted, resulting in a new Anglicised title of William the Conqueror. He then embarked on an uncontested entry into London and coronation at Westminster.

After the Norman conquest, Hertfordshire was used for some of the new Norman castles at Bishop's Stortford and at the royal residence of Berkhamsted and at King's Langley, a staging post between London and the royal residence of Berkhamsted.

The Domesday Book recorded the county as having nine hundreds. Tring and Danais became one, Dacorum, from (Danis Corum or Danish rule harking back to a Viking not Saxon past). The other seven were Braughing, Broadwater, Cashio, Edwinstree, Hertford, Hitchin and Odsey.
As London grew, Hertfordshire became conveniently close to the English capital; much of the area was owned by the nobility and aristocracy, this patronage helped to boost the local economy. However, the greatest boost to Hertfordshire came during the Industrial Revolution, after which the population rose dramatically. In 1903, Letchworth became the world's first garden city and Stevenage became the first town to redevelop under the New Towns Act 1946.

Four towns have between 50,000 and 100,000 residents: Hemel Hempstead, Stevenage, Watford and St Albans. The county town, Hertford, once the main market town for the medieval agricultural county ranks 13th in population today deriving its name from a hart (stag) and a ford used as the components of the county's coat of arms and flag.

Elevations are high for the region in the north and west. These reach over 240m in the western projection around Tring which is in the Chilterns. The county's borders are approximately the watersheds of the Colne and Lea, which flow southwards each accompanied by a canal.

Hertfordshire is at the edge of the London Basin and most of its undeveloped land is agricultural and protected as Metropolitan Green Belt. The volume of intact medieval and Tudor buildings surpasses London, in places in well-preserved conservation areas, especially in St Albans which includes some remains of Verulamium, the town where in the third century AD an early recorded British martyrdom took place. Saint Alban, a Romano-British soldier, took the place of a Christian priest and was beheaded on Holywell Hill. His martyr's cross of a yellow saltire on a blue background is reflected in the flag and coat of arms of Hertfordshire.

The county's landmarks span many centuries, ranging from the Six Hills in the new town of Stevenage built by inhabitants during the Roman Britain centuries, to Leavesden Film Studios. Leavesden filmed much of the UK-based $7.7 Bn box office Harry Potter film series.

The largest sector of the economy of the county is services and it has a large proportion of residents who are City of London commuters. Ten railway lines and three motorways pass through or reach into the county.