Platt was born at Dobcross, West Riding of Yorkshire, the son of Henry Platt who founded Platt Brothers textile machinery manufacturers in 1770.
Foreign other people named John Platt, see John Platt. which by 1820 was based at Werneth area of Oldham. Platt was elected as the first Liberal Mayor of Oldham in 1854 and held office again in 1855-1856. By the mid-1850s his company had established itself as the world"s largest textile machinery manufacturer.
Platt acquired property in Llanfairfechan North Wales in 1857 and rebuild Bryn Y Neuadd as a magnificent mansion.
He was mayor of Oldham again in 1861-1862. He held the seat until his death in Paris at the age of 55.
Platt married Alice Radcliffe in 1842. Their daughter Mary married Thomas Hardcastle Sykes of the Sykes Bleaching Company.
Platt"s younger brother James Platt (1824–1857), was also elected Member of Parliament for Oldham in 1857, but was killed that same year in Greenfield by an accidental discharge of his own gun.
A statue was erected of John Platt in Oldham by David Watson Stevenson in 1878.
19th United Kingdom Parliament. 20th United Kingdom Parliament]
In 1865 he was elected Member of Parliament for Oldham.