The name is derived from Old English welig meaning "willow", referring to the trees that nestle on the banks of the River Mimram as it flows through the village.

[5] Settlement across the area seems to have become established during the Bronze Age according to various recovered artefacts and crop marks left by round barrows and burial mounds from that period.

[6] The Belgae Celtic culture colonised much of South-Eastern England in the 1st century BC, with Welwyn in the area believed to have been settled by the Catuvellauni tribe.

One particular excavation revealed a large Roman cemetery very close to the site of the current church, which itself is known to date back to at least Saxon times (see below).

[9] The massacre on St. Brice's day on 13 November 1002, when the Saxons turned on their newly settled Danish neighbours, is said to have commenced near Welwyn.

The nave of the present church (St Mary's), was built in the 13th century, the chancel arch being the most obvious early structure.

Much later, in the 17th century, as it lies on the old Great North Road, it became an important staging post and a number of coaching inns remain as public houses.

After the Great Northern Railway by-passed the village due to the objections of local landowners, Welwyn became less important.

The 20th century brought major expansion to the area, as estates to the south, west and north of the village were built up.

One writer wrote that Welwyn, a small town in the Maran Valley, can show little of interest beyond many quaint cottages, and the church.

The village stands on the river Maran, 1¼ mile W of the Great Northern railway, and 5 N of Hatfield; carries on shoe-making and wool-stapling; consists chiefly of two well built streets; and has a head post-office,‡ a r. station with telegraph, two hotels, a police station, a good ancient church, two dissenting chapels, a large national school, an education charity, a workhouse, and charities for the poor £26.—The parish includes Woolmer-Green hamlet, and comprises 2,987 acres.

Today the village is the point where the six-lane motorway merges into four lanes and is the site of extensive traffic jams in the evening peak.

There had been extensive plans to widen the whole road through the area to eight lanes, and to upgrade the existing junction to create a long one-way system running the length of the village.

Green Line route 797 used to stop on the by-pass, providing an hourly direct link to areas of North London and the West End, however, the service ceased in September 2016.

Trains are operated by Great Northern and run every 30 minutes Monday to Saturday south to London King's Cross and north to Hitchin and Stevenage, with an hourly service to Letchworth and Cambridge and to Peterborough.

[19][20] In its day it was well admired, with one W. Robinson (writing in "The English Flower Garden", published in 1883), stating that "In the Home Counties there is probably not a better fernery than at Danesbury.

Vincent van Gogh walked from London to visit his sister while she was staying in Welwyn; this is commemorated by a blue plaque on a building on Church Street.

The twinning arrangement was entered into in 1973, as a result of a visit to Champagne-sur-Oise by the headmaster and pupils of St Mary's School, Welwyn, setting up a cultural association which has continued since then.