Thornhill Gardens is bordered by Thornhill Road, Richmond Avenue and Malvern Terrace; it stands on land that was part of Barnsbury Manor, which was sold in 1654, and was primarily used for dairy farming.

By the early 19th century the land was owned by Thomas Thornhill. What became the Thornhill Estate was laid out by the estate surveyor, Joseph Kay, and a number of speculative builders; by 1852 much of it had been built over although small pockets survived, including the area that is now Thornhill Gardens. That piece of land was sold in 1890 by the then-owners, George and John Smith, to the Islington Vestry (the parish of St Mary, Islington), which opened it as a public garden.

On 1 November 1900 the parish – which, in 1889, had been transferred from the County of Middlesex to the newly formed County of London – was abolished. It was replaced by the Metropolitan Borough of Islington.

The garden is protected from building by the London Squares and Enclosures (Preservation) Act 1906.