The eldest son of William Siddeley and his wife born Elizabeth Davenport, J D Siddeley was born in Longsight, Manchester in 1866 and first worked for his father as an apprentice hosier but took night classes in draughting. In 1892, the young bicycle racer and designer was hired as a draughtsman by the Humber Cycle Company. The then managing director of Dunlop picked him out at Humber and hired Siddeley as Dunlop's Belfast sales manager.