The Priory of St John the Baptist at Sandleford was founded for the Augustinian Canons Regular by Geoffrey, Earl of Perch and his wife Maud some time between 1193 and 1202.
In 1274, Maud de Clare, Countess of Gloucester and Hertford made arrangements to refound it as a double house for Fontevrault Benedictine nuns and brothers, but this did not come about. It was dissolved in 1478 and abandoned by the remaining monks after years of mismanagement by a prior. The ownership fell into the hands of the Bishop of Salisbury, and circa 1480 passed to the Dean and Chapter of Windsor. The old priory chapel is the present library. The remains were converted to a country house which was leased out to a number of wealthy citizens during the 17th, 18th and 19th centuries, these included:
- the Pitt family of Strathfield-Saye;
- John and Henry Kingsmill, from c. 1685 and 1706 and 1710, until circa 1715-1717. John Kingsmill, JP (Newbury, 1685) was a younger son of Sir Henry Kingsmill (1587-1625), and husband to Rachael daughter of JP and sometime MP Edward Pitt (c.1592-1643), of Steepleton Iwerne, Dorset and later of Stratfield Saye (which he bought for £4,800 in 1629), by Rachael (d. 1643) daughter of Sir George Morton, Bart., son of Sir William Pitt, kt. 1618, Comptroller of the Household. Their sons Robert and Henry Kingsmill died without issue in 1697 and 1710. George Pitt the brother of Rachel Pitt, Mrs John Kingsmill, married Jane, the daughter of John Savage, 2nd Earl Rivers; Anne Finch, Countess of Winchilsea, daughter of Sir William Kingsmill of Sydmonton Court, was a niece and first cousin.
- William Cradock (died 1736), of Gainford Hall, Gainsford, Durham. He married in 1715 Mary daughter of Gilbert Sheldon of St. Andrew's, Holborn, and bought the lease in 1717, disposing of it in 1729;
- Edward Montagu, grandson of Edward Montagu, 1st Earl of Sandwich, leaseholder from 1730, married Elizabeth Robinson in 1742. Sandleford had been in possession of the first cousin (Rachael Pitt, Mrs. John Kingsmill) of his first cousin (Sir Edward Wortley-Montagu (1678 –1761), the father of Lady Mary Wortley Montagu);
- Matthew Montagu (1762-1831), MP, 4th Baron Rokeby;
- Edward Montagu, the 5th Baron Rokeby, who parted with the lease in 1835, to William Chatteris, and died in 1847;
- William Pollet Brown Chatteris (1810-1889), JP, DL (1852, Berks), educated at Brasenose college, Oxford, and son of a London banker, who eventually bought the freehold, enfranchised the estate, in 1875 from the Dean and Canons of Windsor. His first wife (married 1833) was Anne eldest daughter of Alexander Arbuthnot, Bishop of Killaloe;
- Chatteris' nephew Alpin Macgregor (died 1899) son of Sir John Atholl Macgregor, bart., nephew of Chatteris' second wife, daughter of Admiral Sir Thomas Masterman Hardy;
- Macgregor's niece Miss Agatha Thynne (died 1962), (descended from Thomas Thynne, 2nd Marquess of Bath), wife of the 3rd Baron Hindlip. Her mother (died 1934) and father John Charles Thynne (1838-1918), sometime receiver general to the Dean and Chapter of Westminster, were living at Sandleford Cottage in 1907. Her sister Joan E. M. (1872-1945) was the mother of John Campbell, 5th Earl Cawdor.
- Mrs. Myers, aka Evelyn Elizabeth Myers, who wrote A History of Sandleford Priory, with plates, Newbury District Field Club, Special Publication. no. 1, published between 1900-1931, was tenant from before 1898 to at least 1911.
- Major Aubrey Isaac Rothwell Butler, (1878-27.9.1930), son of Isaac Butler (1839-1917), JP (Sheriff of Monmouth 1910), of Panteg House, Griffithstown, Torfaen, near Newport. It is claimed that the first sheet steel in Britain was rolled in Staffordshire in 1876 from a bloom made in Panteg by Isaac Butler. Aubrey Butler was sometime manager of Baldwin's Ltd branches in Monmouth & Midlands, Baldwins having taken over the family firm, Wright, Butler and Co Ltd, in 1902. Later he was Sheriff of Monmouthshire, 1924, and by the time of his early death was described as formerly of Sandleford Priory and of 13, Porchester Terrace, London.
Edward Haytley's portrait, The Montagu Family at Sandleford Priory, circa 1744.
The present Sandleford Priory is a Grade I listed building in 54 acres (22 ha) of parkland landscaped by Capability Brown. It was erected around the old priory buildings between 1780 and 1786 by James Wyatt, for Elizabeth Montagu, the social reformer, patron of the arts, salonist, literary critic and writer who helped organise and lead the Blue Stockings Society. It was later inherited by her nephew, Matthew Montagu, 4th Baron Rokeby. Her friend Hannah More was there often and described it in 1784.
The house became a school in 1948. St Gabriel's School was founded in 1929 in Mill Hill in London. In 1939 the school was evacuated to West Ogwell Manor in Devon for the safety of the pupils and because their Mill Hill, London site was requisitioned. The school remained in the West Country until 1943 when it relocated to Ormonde House , Newbury. After Easter in 1948 the school moved to Sandleford Priory. There is now a school community of over 460 pupils, 60 teaching staff and 20 support staff. It is associated with traditional Church of England values. This includes Communion once every school term (with exceptions if the student has a valid explanation for not being able to attend) and all students have to take "Religion, Philosophy and Ethics" (Religious Education) for GCSE.