Macmillan completed construction of a wooden bicycle in 1839 that included iron-rimmed wooden wheels, a steerable wheel in the front and a larger wheel in the rear which was connected to pedals via connecting rods.
A Glasgow newspaper reported in 1842 an accident in which an anonymous "gentleman from Dumfries-shire… bestride a velocipede… of ingenious design" knocked over a pedestrian in the Gorbals and was fined five shillings. The gentleman was later identified as Macmillan. A plaque on the family smithy reads "He builded better than he knew."
MacMillan's ingenuity spawned an industry and although there were other lightweight frame builders in Scotland the most prolific and well-known was to David Rattray's Flying Scot and The Scot, which were built in Glasgow between 1928 and 1983. The company manufactured approximately 15,000 frames, which were a "must have" amongst many serious club riders. They were also sold to other parts of the UK and exported to continental Europe, Japan, British Guyana and the USA.