Hitchin is first noted as the central place of the Hicce people mentioned in a 7th-century document, the Tribal Hidage. The tribal name is Brittonic rather than Old English and derives from *siccā, meaning 'dry', which is perhaps a reference to the local stream, the Hiz. It has been suggested that Hitchin was the location of Clofeshoh, the place chosen in 673 by Theodore of Tarsus the Archbishop of Canterbury during the Synod of Hertford, the first meeting of representatives of the fledgling Christian churches of Anglo-Saxon England, to hold annual synods of the churches as Theodore attempted to consolidate and centralise Christianity in England. By 1086 Hitchin is described as a Royal Manor in the Domesday Book: the feudal services of Avera and Inward, usually found in the eastern counties, especially Cambridgeshire and Hertfordshire, were due from the sokemen, but the manor of Hitchin was unique in levying Inward. Evidence has been found to suggest that the town was once provided with an earthen bank and ditch fortification, probably in the early tenth century but this did not last. The modern spelling 'Hitchin' first appears in 1618 in the "Hertfordshire Feet of Fines".
Hitchin is notable for St. Mary’s Church, which is remarkably large for a town of its size. The size of the church is evidence of how Hitchin prospered from the wool trade. It is the largest parish church in Hertfordshire . Most of the church dates from the 15th century, with its tower dating from around 1190. During the laying of a new floor in the church in 1911, foundations of a more ancient church building were found. In form, they appear to be a basilican church of a 7th-century type, with a later enlarged chancel and transepts, perhaps added in the 10th century. This makes the church older than the story (not recorded before the 15th century) that the church was founded by Offa, king of Mercia 757-796.
In 1697, Hitchin (and the nearby village of Offley) were subject to what is thought to have been the most severe hailstorm in recorded British history. Hailstones over 4 inches in diameter were reported
The town flourished on the wool trade, and located near the Icknield Way and by the 17th century Hitchin was a staging post for coaches coming from London. By the middle of the 19th century the railway had arrived, and with it a new way of life for Hitchin. The corn exchange was built in the market place and within a short time Hitchin established itself as a major centre for grain trading.
The latter half of the 20th century has also brought great changes in communication to Hitchin. Motorways have shortened the journey time and brought Luton, a few miles away on the M1, and the A1 (M) even closer. By the close of the 20th century, Hitchin had become a satellite dormitory town for London. Hitchin also developed a fairly strong Sikh community based around the Walsworth area.
During the medieval period, both a priory (Newbigging, now known as The Biggin) and a friary (now known as Hitchin Priory) were established, both of which closed during Henry VIII's Dissolution of the Monasteries. They were never reformed, although The Biggin was for many years used as almshouses.
Girton College—a pioneer in women's education—was established on 16 October 1869 under the name of College for Women at Benslow House in Hitchin, which was considered to be a convenient distance from Cambridge and London. It was thought to be less 'risky' and less controversial to locate the college away from Cambridge in the beginning. The college moved to Cambridge a few years later and adopted its present name, Girton College.