Number 20 Mansfield Street (previously number 4) is on the east side of the street, and the penultimate house at the northern end. It was built for William Grantham (ND), a carpenter who worked on Mansfield Street.

In 1774 the lease was purchased by John Montagu, Viscount Hinchinbrooke (1744-1814), at a rent of £340 for that year. Hinchinbrooke served as MP for Brackley in 1765-68, and Huntingdonshire in 1768-92, and succeeded his father as 5th Earl of Sandwich in 1792. His second wife, whom he married in 1772, was Lady Mary Paulet, the daughter of Harry Paulet, 6th Duke of Bolton, and Adam's patron at Hackwood Park, Hampshire, and Bolton House, Southampton Row. It is possibly through this connection that Hinchinbrooke became aware of the scheme in Mansfield Street. Hinchinbrooke remained in residence at number 20 until 1783. The house then passed through various hands, including those of the reformer, Charles Stanhope, 3rd Earl of Stanhope in 1787-95.

Although the house suffered a fire in 1794, number 20 is one of the surviving houses, and is the only house on Mansfield Street that was not stuccoed on the ground storey during the nineteenth century. Adam's chimneypieces, stairwell, and ceilings in the drawing rooms and hall all survive. It is important to note, however, that only the back drawing room ceiling is documented with an extant Adam drawing. Moreover, Adam's design for the front drawing room ceiling was unexecuted in favour of a copy of his design for the front drawing room ceiling in Lord Scarsdale's house at number 15.

Some alterations have been made to the house, including a nineteenth-century Adam-style entrance, and an unexecuted Adam ceiling design made for the ante room of number 22 was installed in the front ground floor room (dining room) of number 20 in the 1950s.

All of the houses on the east side of the street were purchased after the Second World War by the British Employers Confederation, and remained their headquarters until 1997. In 1998 a 150-year lease was acquired for all four buildings from the Howard de Walden estate by a developer who separated the houses, and sold the leases individually. Number 20 is now a private residence.

Literature:
See Mansfield Street scheme notes.

Frances Sands, 2013