He was the eldest son of John Bennet, watchmaker, of Greenwich. Educated at Lewisham Grammar School, in 1846 he established his own business at 65 Cheapside, in the City of London. He was a common councillor for the ward of Cheap from 1862-89.
In October 1872 he was elected to the London School Board to fill a casual vacancy in the representation of the City of London.
Although he stood down from the school board at the next elections in 1873, he returned to serve a three-year term from 1876-79.
In 1877 he was elected an alderman for Cheap ward, but was declared to be not of fit character to hold the office.
In 1889 he retired from business and the common council.
He died at St. Leonards-on-Sea on the Sussex coast, aged 82. He was described in an obituary as "a man of strong character, very eccentric, and one of the most familiar figures in London".
- Sir John Bennett Dead. Famous English Watchmaker Passes Away in London. New York Times (6 July 1897).
- Edward Irving Carlyle. Sir John Bennett (1814-1897) in Bennett, William Cox. Dictionary of National Biography, 1901 supplement.