While much of this part of Middlesex developed in the interwar period, the larger institutions (such as convents and schools) and many landowners decided not to sell land to builders. After World War Two the establishment of the green belt restricted the use of land in Mill Hill Village and along the Folly Valley between Mill Hill and Totteridge. Mill Hill has a number of interesting buildings as a result.
One of the oldest buildings in the village is Rosebank. In 1678 Richard Haley, a local Quaker, had a meeting-house built, which was active until the early 18th century. It is one of a number of weatherboard cottages, a style that was once common in this part of Middlesex. The Nichol Almshouses, at the top of Milespit hill, have an inscription which reads: "These six Almes Houses were erected in ye year of our Lord 1696 a the sole charge of Thomas Nicoll of this Parish Gent."
Mill Hill did not always have an Anglican church. Until the 1820s parishioners had to go to St Mary's Hendon to worship, but in the 1820s it was clear that the district would require a chapel. William Wilberforce became the central benefactor in 1827. St Paul's, which is in the modest Commissioner Gothic style, was completed in 1830, and consecrated in 1833.
In 1869 Father Herbert Vaughan (later Cardinal Vaughan), started a missionary college at Holcombe House. The college moved to new buildings, of Lombardo-Venetian style, in 1871. The tower is 100-foot-high and surmounted with a 14-foot-high statue of St Joseph made by Baumeister of Munich and gilded with copper.
In 1936 the Metropolitan Borough of Paddington established a cemetery in Milespit Hill, where there is a small burial ground for Dutch servicemen and women, and the graves of Bebe Daniels, film star, and Billy Fury, the 1950s pop singer.
By far the largest building is the National Institute for Medical Research. It was constructed between 1937 and 1939, but was not officially opened until 1950. During the war the building was used by the Women's Royal Naval Service.