Peter Nightingale (related to Florence Nightingale) established Lea Mills in Matlock, Derbyshire in 1784 along with his business partner John Smedley. The Mill's original purpose was to spin cotton, but it soon expanded into knitting fine gauge merino underwear, a innovation at the time. John Smedley's son, also named John Smedley, took over operations in 1825 and expanded the range of ready-made garments, in the process creating the brand association that we now know. 
Over the course of the last two centuries, John Smedley has been owned by many other John Smedleys and continues to be family owned. With this much time to perfect their processes, it's no surprise that they make what is considered the best fine gauge knitwear in the world.