John Benn-Walsh was the only son of Sir John Benn-Walsh, bart., of Warfield Park in Berkshire and Ormathwaite in Cumberland. His father was the son of William Benn of Moor Row, Cumberland, a member of an old north-country family. He had married, in 1778, Margaret, daughter of Joseph Fowke of Bexley, Kent, by his wife Elizabeth, daughter of Joseph Walsh, Governor of Fort St. George. On 4th April 1705, he had assumed the surname and arms of Walsh by Royal license, in compliance with the will of his wife's uncle, John Walsh, son of Joseph Walsh. He was later created a baronet, on 14th June 1804, sat for Bletchingly 1802-6 and died on 7th June 1825.

John Benn-Walsh Junior was educated at Eton and matriculated at Christ Church, Oxford, on 3rd December 1816. Entering Parliament for the borough of Sudbury in 1830, he represented that constituency in the tory interest in three parliaments until December 1834. An ardent politician and an able writer, he published several pamphlets on parliamentary reform. In January 1835, Sir John contested the county of Radnor, but was defeated by a small majority. At the next general election, following the accession of Queen Victoria in 1837, he was an unsuccessful candidate for Poole in Dorset, but, the following March, was again returned at a by-election for Sudbury. Two years later, however, he accepted the Chiltern Hundreds and was returned, on 10th June 1840, without opposition for Radnorshire, which he afterwards represented for nearly twenty-eight years, the only occasion on which his re-election was challenged being in 1841, when he defeated Lord Harley.

Walsh was JP and DL for Berkshire and served as High Sheriff for that county in 1823. Being lord of the manor of Trewerne in Radnorshire and the owner of considerable property there, he was also JP for that county and High Sheriff in 1825 and, on 11th August 1842, was sworn in as Lord-Lieutenant and Custos Rotulorum of Radnorshire. On 16th April 1868, he was raised to the peerage as Baron Ormathwaite.

Owing to his advancing years, he resigned the Lieutenancy of Radnorshire in favour of his son, the subsequent Lord Ormathwaite, who received the appointment on 19th April 1875. Ormathwaite died at his seat, Warfield Park, near Bracknell in Berkshire, on 3rd February 1881. He married, on 8th November 1825, Jane, youngest daughter of George Harry Grey, 6th Earl of Stamford & Warrington. By her, he had two sons and two daughters, and was succeeded by his eldest son, Arthur.

Ormathwaite was author of some able pamphlets, of which the principal were: The Poor Laws in Ireland (1830), Observations on the Ministerial Plan of Reform (1831), On the Present Balance of Parties in the State (1832), Chapters of Contemporary History (1836), Political Back-Games (1871), Astronomy and Geology Compared (1872) and Lessons of the French Revolution, 1789-1872 (1873).