Ambrose St John (1760–1822) of Prior Park, Berkshire and The Close, Winchester, Hampshire, was a Member of Parliament for Callington in Cornwall and an officer in the Worcestershire Militia.

In April 1796 he was commanding a 300-strong detachment of the regiment in Truro when around 3000 striking Cornish tin miners approached the town.

Under the direction of the local magistrates, St John and his militiamen confronted the rioters, and when they would not disperse after the Riot Act was read, the militia fired their light field guns over their heads and drove them away at the point of the bayonet, the ringleaders being arrested.

[1][3] He married Arabella Hamlyn (d. 1805),[1] only daughter of Sir James Hamlyn, 1st Baronet (1735–1811) (born "James Hammet") of Clovelly Court in Devon, and of Edwinsford, Carmarthenshire, Wales, a Member of Parliament for Carmarthen 1793–1802 and Sheriff of Devon 1767–8.

By Arabella he had issue two sons and five daughters,[1] including: He died at Douglas, Isle of Man, on 29 November 1822.