Global database of Hertfordshire Real Estate Agents. Hertfordshire, administrative and historic county of southern England, adjoining Greater London to the south. The administrative county and the historic county cover slightly different areas. The administrative county comprises 10 districts: East Hertfordshire, North Hertfordshire, Three Rivers, and Welwyn Hatfield; the boroughs of Broxbourne, Dacorum, Hertsmere, Stevenage, and Watford; and the city of St. Albans. The Potters Bar area in Hertsmere borough lies outside the historic county of Hertfordshire and within the historic county of Middlesex. The historic county of Hertfordshire, however, includes much of northern and central Barnet borough in Greater London. Hertford, in south-central Hertfordshire, is the county seat.