Apart from NPG 1940, the only other individual portrait is by Joseph Highmore, signed and dated 1735, now in the Palace of Westminster. 
Formerly owned by Godfrey Williams it was sold Christie's, 4 October 1946, lot 103, as 'The Right Hon. Arthur Custon' - an obvious misreading. The group of MPs, with Onslow and Walpole as principal figures, inscribed Done by Sir James Thornhill then a Member of the House of Commons, 1730
and engraved by A. Fogg, 1803, carries a MS note by the sitter that 'some of the figures were done by Mr Hogarth'. 
The original, presumably owned by Onslow, is still at Clandon. Reynolds was consulted about a copy for Wimpole c
.1784, referred to in a letter dated 29 August 1784 from the 2nd Earl of Hardwicke to the then Earl of Onslow: 'I return you my best thanks for yr obliging compliance with my request abt Sir James Thornhills Picture. I will with yr Lordships permission consult Sir J. Reynolds abt a proper copy [?] for it ... I think it wd be advisable to have the size ... the same with that of the original.' 
The portrait, listed at Wimpole 1804 
and lent by the Earl of Hardwicke to the Second Exhibition of National Portraits, South Kensington of 1867 (285), was sold Christie's, 24 February 1939, lot 52. Another good copy possibly c
.1800-10, is in the Palace of Westminster. 
A portrait of the sitter is included in Pine's engraving
of the House of Commons in session c
.1741 and 'a curious one ... in his seat at St Margaret's Church, Westminster' by A. Walker formed the frontispiece to Wilson's Ornaments of Churches considered, published 1761. 
A memorial, possibly by Scheemakers, 
erected by the sitter's son, is in Trinity Church, Guildford.
1) R. J. B. Walker, A Catalogue of paintings ... in the Palace of Westminster
, 1960, II, p 83 (159); the literary portraits and others were sold from Clandon, Christie's, 23 June 1827, and the sitter's library and autograph notes at Sotheby's, March 1885.
2) C. E. Vulliamy, The Onslow Family
, 1953, pp 99-100.
3) Clandon archives, kindly communicated by R. St John Gore, 3 August 1972.
4) G. P. Harding, List of Portraits ... in Various Mansions of the United Kingdom ...
.1804, II, p 227.
5) R. J. B. Walker, A Catalogue of paintings ... in the Palace of Westminster
, 1960, II, p 134 (190).
6) Dictionary of National Biography
, XIV, p 1112.
7) So attributed by Mrs Esdaile, verbal.This extended catalogue entry is from the out-of-print National Portrait Gallery collection catalogue: John Kerslake,
Early Georgian Portraits, Her Majesty's Stationery Office, 1977, and is as published then. For the most up-to-date details on individual Collection works, we recommend reading the information provided in the Search the Collection results on this website in parallel with this text.