John Walsh, the secretary to Lord Clive and man of science, was the son of Joseph Walsh, Governor of Fort St. George, by his wife Elizabeth, daughter of Neville Muskelyne (1663-1711) of Purton in Wiltshire. Neville Maskelyne, the Royal Astronomer, and his sister, Margaret Maskelyne, who married Robert, 1st Baron Clive, were his first cousins. Like many of his relatives, Walsh entered the service of the East India Company (in 1742)  and became paymaster of the troops at Madras. In 1757, Clive appointed Walsh his private secretary and he acted in this capacity throughout the campaign in Bengal of that year. Two years later, Clive commissioned him to return to London and lay before Pitt his project for reorganising the administration of Bengal, a subject of which he said Walsh was "a thorough master." In a letter, dated 26th November, Walsh gives Clive an account of his interview with Pitt.

Walsh now settled in England with an amassed fortune estimated at £140,000,  purchasing, in 1761, the manor of Hockenhull in Cheshire. He sold it before long and acquired Warfield Park, near Bracknell, in Berkshire, in 1771. On 30th March 1761, he was returned to Parliament for Worcester, his object being mainly to form a parliamentary interest in Clive's support. However, he also, apparently, became a friend and supporter of John Wilkes - the Buckinghamshire gentleman and political opponent of the Earl of Bute - particularly in the infamous affair of the libeling issue 45 of his 'North Briton' newspaper. He retained his seat till 1780 and much of his correspondence with Clive is printed in Malcolm's 'Life of Clive' (1836). He also corresponded with Warren Hastings, but quarrelled with him in 1781 because of the dismissal of his nephew, Francis Fowke, from his post at Benares.

Walsh was returned to parliament for the city of Worcester on 30th March 1761 and retained his seat until 1780. He died, unmarried, some fifteen years later, on 9th March 1795 in London, at his residence in Chesterfield Street, although he was buried in the parish church in Warfield. He left his property, including Warfield Park, to Sir John Benn, who had married, in 1778, Margaret, daughter of Walsh's sister, Elizabeth. Benn assumed, in accordance with the provisions of the will, the additional name of Walsh, and was father of Sir John Benn-Walsh, 1st Baron Ormathwaite.