In 1723, some of the Middlesex part of the parish broke away to become the parish of St. George the Martyr. In 1767 the remainder of the Middlesex part of the parish was added to St George the Martyr parish, becoming the parish of St Andrew Holborn above the Bars with St George the Martyr ("above the Bars" meaning outside the gates of the City of London)--and was commonly named Holborn. In 1889 the parish was transferred from Middlesex to the newly-created County of London, and in 1900 became part of the Holborn Metropolitan Borough. Holborn Metropolitan Borough was abolished in 1965, becoming part of the London Borough of Camden.
Further description of Holborn Metropolitan Borough, including a map of all the parishes within it, will be found in Wikipedia.