The son of Henry Ashurst Ashurst was a member of the Corporation of London from 1678, and an alderman from 1687; he was knighted in 1687, and was Sheriff of London in 1691–1692 and Lord Mayor of London in 1693–1694.
A Whig, he represented the City of London in Parliament for three separate periods between 1689 and 1710; he was generally regarded as one of the Country Whigs, but voted with the Court Whigs against the Disbanding Bill in 1698-9.
He was a good friend of Edmund Calamy and was a nonconformist like the rest of his family, so unsurprisingly he was an active supporter of the Glorious Revolution and sat in the Convention Parliament (1689).
Sir William married Elizabeth the daughter of Robert Thompson. When in the country they lived in a Queen Anne style red-brick mansion he built in the outer bailey of Hedingham Castle after his purchase of the castle in 1693.
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- Walcott, Robert (1956). English Politics in the Early Eighteenth Century. Oxford University Press.
- Hayton, David (1987). The Country Party in the House of Commons 1698-1699: a Forecast of the Opposition to a Standing Army?. Parliamentary History 6. pp. 141–63.