A Leeds businessman, born in 1863 at Huddersfield, he was a circuit steward and, from 1900, was a trustee of Crossgates WM chapel and the treasurer of its building committee in 1907. He was a West Riding County Councillor and alderman and a Freemason.
The owner of a Leeds wholesale drapery warehouse, in 1884 he 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 approached his Chief Cashier, Tom Spencer (d.1905, aged 53), 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. He died at Whitby in 1937.