The parish of Holwell lies north of Hitchin, west of the A600 to Bedford. Holwell is the only settlement in the parish. It lay in Bedfordshire until 30 September 1897 and the detached part of Cadwell became incorporated into Ickleford along with the part of the parish known as Snailswell that extended east of the Bedford Road (A600) during the 1990s.
Holwell’s entries in Domesday Book; the first entry is for Cadwell, now in Ickleford parish, and the second is for Holwell proper. Both places were in Bedfordshire until 1894, when they were transferred to Hertfordshire.