He entered the English East India Company at the age of fifteen and eventually became Clive's private secretary. During the 1757 Plassey campaign against the Nawab of Bengal, Siraj Ud Daulah, John Walsh was awarded £56,000 in prize money. Upon his return to England in 1759, his fortune was estimated at £147,000, and he quickly sought to purchase the necessary trappings of aristocratic power in eighteenth century Britain: land and political influence. In late 1764, Walsh purchased the large estate of Warfield Park, near Bracknell in Berkshire and spent the next two years doing it up. He was MP for Worcester from 1761 to 1780. He continued to serve Robert Clive, or 'Clive of India' as he became known, and attempted to form a parliamentary interest in his favour.
In later life, John Walsh's interests were scientific, with a particular interest in electric fish. He was elected a Fellow of the Royal Society in 1770 and awarded their Copley Medal in 1773 for a paper on the electrical properties of torpedo fish.
Upon his death in 1795, Sir John Walsh, as he was then known, left his fortune to his niece, Margaret Walsh, and her husband, John Benn, on the condition that they change their surname to Benn-Walsh. With his own fortune of £80,000 made in India while Assistant to the Resident of Benares, his brother-in-law Francis Fowke in the 1770s, John Benn-Walsh had become a very wealthy man and went on to inherit extensive estates in Warfield, Buckinghamshire, in Radnorshire, and in Ormathwaite, Cumberland and be created Baron Ormathwaite.