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

The area has a history dating back to 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, Hertfordshire adapted quickly to the Roman way of life, and one of the new towns, Verulamium, became the third largest town in Roman Britain. After the Romans left Britain, the Anglo-Saxons occupied the area, creating their own towns, including the county town of Hertford.

The Norman conquest in 1066 reached its climax at Berkhamsted where William the Conqueror accepted the final Saxon surrender. 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.

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, and much of the area was owned by the nobility and aristocracy, and 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.

In 1965 under the London Government Act 1963 East Barnet Urban District and Barnet Urban District were abolished and their area was transferred to Greater London to form part of the present-day London Borough of Barnet. At the same time the Potters Bar Urban District of Middlesex was transferred to Hertfordshire.

From the 1920s until the late 1980s, the town of Borehamwood was home to one of the major British film studio complexes, including the MGM-British Studios. Many well-known films were made here: for example, all the Harry Potter films were made at Leavesden Film Studios near Watford.

In early December 2005 the 2005 Hemel Hempstead fuel depot explosions occurred at the Hertfordshire Oil Storage Terminal.

In 2012, the Hertfordshire town of Broxbourne will host the canoe and kayak slalom events of the 2012 Summer Olympic Games.