Moreton Frewen was the fifth son of Thomas Frewen, MP for South Leicestershire. He was educated at Trinity College, Cambridge, gaining 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 Clara Jerome (1853-1924) 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, where Frewen ran up ever increasing debt, earning the sobriquets 'Mortal Ruin' and 'the splendid pauper'.
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.
From 1910 to 1911 he was All-for-Ireland League MP. for North East Cork (he resigned 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 whon he was not on good terms) was the second wife of Sir Edward Carson.
A daughter was the sculptress and writer Clare Frewen Sheridan.