Platt was elected as the first Liberal Mayor of Oldham in 1854 and held office again in 1855–56. By the mid-1850s his company had established itself as the world's largest textile machinery manufacturer.[2] Platt acquired property in Llanfairfechan North Wales in 1857 and rebuild Bryn Y Neuadd as a magnificent mansion.[3] He was mayor of Oldham again in 1861–62. In 1865 he was elected Member of Parliament for Oldham. He held the seat until his death in Paris on 18 May 1872, at the age of 55.[4]
Platt married Alice Radcliffe in 1842. She died at Oakleigh, Leamington, on 19 December 1902, aged 79.[5]
Their second son Samuel Radcliffe Platt (1844–1902) became head of the firm Platt Brothers, and was High Sheriff of Lancashire in 1897.[6]
Platt's younger brother James Platt (1824–1857) was also elected MP for Oldham in 1857,[1] but was killed that same year in Greenfield at Ashway Gap on Saddleworth Moor by an accidental discharge of the gun of Josiah Radcliffe, the Mayor of Oldham.[7]