The earliest reference I could find to a spring seat in a vehicle was in William Felton's [*A Treatise on Carriages*](https://archive.org/details/atreatiseoncarr00feltgoog) (published 1794), in which he describes (on page 144):

> it [the coach-box] is fixed on to the top iron-work with bolts, having a cradle, the same as the others, for the seat; they sometimes hang upon springs, and are made with a head and knee-flap the same as to a one-horse chaise; their use is to make the situation of the servant more comfortable, and more secure from danger, by travelling on bad roads

This source seems to take the existence of the spring seat for granted, so it seems to already be implemented at this time, meaning the spring seat may have been around for longer. However, the [Gold State Coach](https://en.wikipedia.org/wiki/Gold_State_Coach) (commissioned in 1760) does not have a sprung seat, so the innovation probably occurred at some point between 1760 and 1794.