He was the fifth son of Thomas Frewen, MP for South Leicestershire, and was educated at Trinity College, Cambridge, where gained his BA in 1877. He emigrated to Wyoming during the cattle boom in the 1870s and 1880s. A charming if financially incompetent adventurer from an English landed gentry family known for reckless financial and political schemes, he managed to marry Clarita "Clara" Jerome (1851–1935) in 1881, daughter of the New York financier Leonard Jerome, and sister to Lord Randolph Churchill's wife Jennie. They settled together on a huge Wyoming ranch, The Prince of Wales Ranch' where Frewen built an enormous log Lodge/Castle later destroyed by fire and ran up ever increasing debt, earning the sobriquets 'Mortal Ruin' and 'the splendid pauper'. His laterally descending family, the Martin's formerly of Charley Hall, Leicestershire however, refer to him to this day not as Mortal Ruin but as 'Immortal Ruin' as he ran through two family fortunes before being granted a remittance and 'encouraged' by family to emigrate to America. 
Returning to the United Kingdom – where he owned homes in London and Cork – Frewen served as Vice President of the Imperial Federation League. He wrote on tariff reform and other economic matters, and was an advocate of bimetallism.
He was elected unopposed at the December 1910 general election as an All-for-Ireland League MP for North East Cork, taking his seat in the House of Commons of the United Kingdom of Great Britain and Ireland. He resigned on 5 July 1911 because his seat was needed for Healy and because of his reactionary public statements: his opposition to the Parliament Bill to remove the legislative veto of the House of Lords was proving a political liability. Later he signed the British Covenant in support of Ulster, while continuing to engage in political intrigues. He was a brother-in-law of Lord Randolph Churchill and Sir John Leslie of Glaslough. His niece (with whom he was not on good terms) was the second wife of Sir Edward Carson.
He left two sons, Hugh Moreton Frewen, and Captain Oswald Moreton Frewen, Royal Navy (Retired). A daughter was the sculptress and writer Clare Frewen Sheridan. Another daughter, Jasmine, died at birth.
- The economic crisis, 1888
- Melton Mowbray, and other memories, 1924
- Who was who 1916–1928 (1929)
- "Frewen, Moreton (FRWN872M)". A Cambridge Alumni Database. University of Cambridge.
- Woods, L. Milton: Moreton Frewer's Western Adventures (1986)
- Maume, Patrick: The long Gestation, Irish Nationalist Life 1891–1918, "Who’s Who" p. 228, Gill & Macmillan (1999) ISBN 0-7171-2744-3
- Kehoe, Elizabeth: The Titled Americans: Three American sisters and the British aristocratic world into which they married, Atlantic Monthly Press (2004), ISBN 0-87113-924-3
- Andrews, Allen: The Splendid Pauper (1968)
- Mrs. Selina Margaret Clay (nee Martin) of Victoria BC, youngest daughter of Rev. John Martin of Charley Hall, Leicestershire (1891-1980) and 1st cousin to Moreton Frewen
- Brian M. Walker, ed. (1978). Parliamentary election results in Ireland 1801–1922. Dublin: Royal Irish Academy. p. 178. ISBN 0-901714-12-7.
- Department of Information Services (9 June 2009). "Appointments to the Chiltern Hundreds and Manor of Northstead Stewardships since 1850". House of Commons Library. Retrieved 30 November 2009.[dead link]
- Archival material relating to Moreton Frewen listed at the UK National Archives
- Hansard 1803–2005: contributions in Parliament by Moreton Frewen
|Parliament of the United Kingdom|
|Member of Parliament for North East Cork
December 1910 – July 1911