Grenfell studied at Truro Grammar School and then joined a branch of his father’s business in London before becoming principal manager of ‘Thomas Williams of Llanidan’ a brass and copper producer on Anglesey.  He was governor of the Royal Exchange Assurance Company from 1829 to 1838.

On Thomas Williams’ death, Grenfell replaced him as Member of Parliament for Great Marlow before switching constituency to Penryn.

In Parliament, he was a strong supporter of William Wilberforce’s anti-slavery and anti-slave trade campaigns. 

Grenfell married his cousin,  Charlotte Granville (1765–1790), who died in childbirth and he subsequently married Georgiana St Leger (1775–1818) with whom he had five daughters including Frances Eliza (1814–1891) who married Charles Kingsley (June 12^th), a close friend of C.A. Johns (June 28^th).