A Leeds businessman, born on 1 February 1863 at Huddersfield,where his father Benjamin, a chemist and druggist, was a member of Queen Street Wesleyan chapel. On leaving school Isaac became a draper in Huddersfield, moving to Leeds in the early 1880s to set up a wholesale and retail business in Kirkgate. He was Trust Treasurer of Crossgates WM chapel from 1897 until his death, treasurer of the building committee from 1907, and Circuit Steward in the Leeds (Richmond Hill) Circuit. He was elected to Barwick-in-Elmet parish council in 1898 and the West Riding County Council in 1910. He became a JP in 1915 and a County Alderman in 1923. He was also a Freemason.
In 1884 Isaac lent £5 to Michael Marks (1859-1907) a Russian-born Jew and Polish refugee, to start a business as a peddler, initially selling his wares in the Yorkshire Dales. Marks came to Leeds in 1882, unable to speak English, and in 1884 opened a stall in Kirkgate Market, 'Marks Penny Bazaar'. In 1894 Dewhirst declined an offer to go into business with Marks but suggested he approach his Chief Cashier, Tom Spencer (1851-1905), with the result that the firm of Marks and Spencer was established. For over a century Dewhirst's firm was a supplier to Marks & Spencer.
Isaac Dewhirst married Clara Backhouse at Manston St James's on 5 September 1887. She was for many years the only female trustee at Crossgates and she represented Scholes on the Tadcaster Rural District Council.
Isaac died at Robin Hoods Bay, near Whitby on 16 May 1937, leaving an estate equivalent to £3.5 million at 2025 values..