He was appointed High Sheriff of Suffolk in 1860. He was elected to the House of Commons as one of the two Members of Parliament (MPs) for the Western division of Suffolk at a by-election in October 1875, and held the seat until the constituency was abolished at the 1885 general election.
Thornhill married Katherine Edith Isabella Hodgson, daughter of Richard Hodgson-Huntley, of Carham Hall, Northumberland, by his wife Catherine Moneypenny Compton, daughter of Anthony Compton, of Carham Hall. Lady Thornhill was in January 1902 granted permission to take the surname and arms of Compton combined with Thornhill, for herself and her issue. He was succeeded by their son Anthony John Compton-Thornhill.
- "House of Commons constituencies beginning with "S" (part 6)". Leigh Rayment's House of Commons pages. Retrieved 23 November 2009.
- Craig, F. W. S. (1989) . British parliamentary election results 1832–1885 (2nd ed.). Chichester: Parliamentary Research Services. pp. 463–464. ISBN 0-900178-26-4.
- "no. 25499". The London Gazette. 11 August 1885. p. 3701.
- "no. 27401". The London Gazette. 28 January 1902. p. 581.
- Hansard 1803–2005: contributions in Parliament by Sir Thomas Thornhill, Bt
|Parliament of the United Kingdom|
Fuller Maitland Wilson
|Member of Parliament for West Suffolk
1875 – 1885
With: William Parker 1875–1880
William Biddell 1880–1885
John George Sheppard
|High Sheriff of Suffolk
Edward Robert Starkie Bence
|Baronetage of the United Kingdom|
(of Riddlesworth Hall and Pakenham Lodge)
Anthony John Compton-Thornhill
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